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US Re-examination - Comparison with EPO Opposition

Re-examination is a process available in the US Patent and Trademark Office to provide for re-evaluation of an issued patent based on prior art not considered by the examiner in the initial patent issuance.  Here we provide a brief review of that process and we present a table comparing the process with EPO opposition.

A request for a re-examination can be filed at the US Patent and Trademark Office (PTO) by anyone at anytime during the period of enforceability of a patent.   The request must raise a substantial new question of patentability.  The scope of re-examination is limited to prior art patents and publications. Requests for re-examination are often filed by third parties, who are already involved in an infringement lawsuit concerning the patent at issue. By filing for re-examination, such parties seek to invalidate the patent while keeping legal fees low. If the judge agrees, the trial proceedings may be put on hold pending the outcome of the re-examination.

Re-examination can also be used by a patent holder when an alleged infringer raises prior art as a reason for invalidity as a defence in an infringement or declaratory judgement action. The patent holder can amend claims to make the claims narrower in light of the prior art. The patent holder will make every effort to preserve the patent and to ensure that the re-examined patent will read on the alleged infringer's accused device.

Once the re-examination has been concluded, a "certificate of re-examination" is issued. The certificate makes any corrections to a patent as are required under the re-examination. If all the claims in the patent are rejected, the patent is nullified. The decision of the USPTO is entitled to deference, just as was the original decision to grant. 

A typical re-examination takes about two to three years, and the initial determination as to whether a substantial new question of patentability exists is usually made within three months of filing.

Proceedings of all re-examinations are made available to the public on the USPTO's public PAIR (Patent Application Information Retrieval) web site.

Ex parte re-examination

An ex parte re-examination is initiated by any member of the public, but the requester no longer actively participates in the proceedings. The correspondence is strictly between the examiner and the patent owner. 

If the PTO decides that there is no "substantial question" of patentability, the application for re-examination is rejected, the matter ends and the decision cannot be appealed. The patent holder has the benefit of the presumption of validity in light of the new prior art in any subsequent court proceeding. 

If the PTO decides that there is a substantial question of patentability, the re-examination will proceed.  A new examiner is assigned to the case, and the patent goes through another examination similar to the examination it received the first time around. The patent holder is given at least two months to present any arguments or amendments concerning the new prior art. Where there is a third party, the USPTO will give the third party at most two months to provide a reply to the arguments and amendments offered by the patent holder. At this point, the third party's participation in the re-examination stops.  The re-examination then proceeds just as an initial examination, with office actions, responses, and interviews with the examiner. The third party can do no more than monitor the proceedings on the USPTO's public PAIR website. He or she is not privy to any conversations between the examiner and the patent owner.

If any claims are rejected in light of the new questions raised, then the patent owner can narrow or cancel those claims. The patent owner can also submit new claims, provided they are not broader than the claims in the original patent. If the examiner makes a rejection "final", the patent owner can appeal the decision to the Board of Patent Appeals and Interferences at the USPTO. The patent owner can file an appeal to the Court of Appeals for the Federal Circuit and even to the US Supreme Court, if permitted.

The third party that requested the re-examination has no right of appeal.

The patent holder has a substantial advantage in this process over the third party, because only the patent holder can participate in the re-examination process to any significant extent.
One advantage of an ex parte re-examination is that the third party need not identify itself. 

On all grounds other than cost, the ex parte re-examination process, particularly for third parties, offers a very poor alternative to a court action to challenge a patent and is heavily weighted in favour of the patent holder through the denial of participation to the third party in the re-examination.

EPO OppositionUS Ex-Parte Re-ExaminationUS Inter-Partes Re-Examination
AvailabilityOnly during 9 month window following grantAt any time after grantOnly for patents granted an applications filed on or after 29 November 1999
ConfidentialityCan be filed by a straw manReal party of interest need not be revealedReal party of interest must reveal itself
Available GroundsMost grounds (i.e. not patentable, not sufficiency described, subject matter added by amendment). Mere lack of clarity not ground of opposition.Must be based on printed prior art. Not on evidence of use or sale. May not be cumulative to information already considered.Must be based on printed prior art. Not on evidence of use or sale. May not be cumulative to information already considered.
CostInexpensiveModerately expensiveExpensive
Third Party InvolvementAll through to oral proceedingsThird party's involvement ceases after response to Patentee's initial defence

Severe restriction on again raising same issues before a Court. Extends to issues that could have been raised.

Any determination of fact cannot later be challenged.
Third Party Success Rate


  • 37% revoked
  • 32% upheld in amended form
  • 31% upheld as granted


  • 10% revoked
  • 64% partially upheld
  • 26% upheld as granted


  • 10% revoked
  • 64% partially upheld
  • 26% upheld as granted

AppealOne level (Board of Appeal)Patentee can appeal to the BPAI and to the Federal Circuit.

Third party has no right of appeal
Patentee and third party can (in theory) appeal to the BPAI and to the Federal Circuit

Inter partes re-examination

Inter partes re-examinations are initiated by a third party who continues to participate in the proceedings. The procedure is relatively new and is only available for patents that are issued from applications filed on or after November 29, 1999.

In inter partes re-examination, each time the patent owner files a response to a PTO action on the merits, such as an office action, the third party is entitled to file a response within thirty days.
The inter partes re-examination process is intended to offer a low cost alternative to litigation to challenge the validity of a U.S. patent based on prior art. 

Unlike an ex parte re-examination request, however, an inter partes re-examination request must identify the real party in interest. 
The initiation of an inter partes re-examination has several substantial effects. 

 a.  First, other litigation is stayed on request of the patent owner (but not the third party) unless the court determines that justice will not be served. This evinces a bias toward resolving issues of prior art patentability in the PTO, not in the courts.

b.  Second, the third party requester is estopped from asserting invalidity of the patent in any civil action based on the prior art considered in the re-examination or prior art that could have been raised at that time. The effect is greater than the presumption of validity. The third party cannot even raise the issue.

c.   Third, the patent owner, third party and their privies are prohibited from requested further re-examinations during the period that an inter partes re-examination is proceeding.  This serves to prevent a third party from abusing the patent owner by showering the patent owner with separate requests for inter partes re-examination.

d.   Fourth, a third party is prohibited from requesting a new inter partes re-examination based on any matter which the party could have raised in the initial proceeding.

e.   Fifth, a party requesting re-examination is estopped from challenging any fact determined during the re-examination.
Third-party participation helps explain the greater success seen in invalidating patent claims through inter partes re-examination than in ex parte re-examination (see table).

A significant drawback of an inter partes re-examination is that, following a final decision (including any appeals), the third party, and those in privity with it, are precluded from litigating issues that were raised or could have been raised in the re-examination.
It is said that the goals of inter partes re-examination of being faster, better and cheaper that the district courts in assessing patentability based on patents and printed publications have failed so far in practice.

Joint Defence Group

Typically, in a multi-defendant litigation, the defendants will create a joint defence group.  One or more defendants may be appointed to file an inter partes re-examination request and shelter other members from the estoppel provisions [35 U.S.C. §§ 315(c) and 317(b)].  It remains unclear how far these provisions extend. 


By contrast, third party US re-examination is open at any time but has a low success rate, whereas EPO opposition has a limited time window and a higher success rate. Although EPO opposition can be initiated only during a brief window, it represents a cheap and effective way of knocking out nuisance patents in Europe and can have a profound effect on license negotiations worldwide.  Keeping a watch on a competitor patent for grant of the patent and opening of the opposition window is simple and inexpensive and an Opponent can remain anonymous.  Missing the window can be a matter for regret later.  Contact us at an early stage in any competitive battle to take a look at the state of progress of a competitor's European applications and patents so as not to miss this valuable opportunity.