According to previous case law of the Federal Court of Justice (BGH) concerning declarations by patent proprietors, there has never been an institute such as a “file estoppel”. In general, narrowing or limiting declarations by the patent proprietor have only been considered relevant for determining the extent of protection of a patent in infringement actions when having been made in preceding opposition proceedings. In such cases, it has been ruled that it may violate the principles of good faith from the point of view of the impermissible exercise of rights if the patent proprietor declares in opposition proceedings that he does not seek patent protection for a certain embodiment, but then nevertheless asserts claims from the patent against the opponent on account of this embodiment (see BGH, judgment of June 5, 1997 – X ZR 73/95 – Weichvorrichtung II). The same applies to declarations in nullity proceedings (OLG Düsseldorf, judgment of November 10, 2011 – 2 U 40/11 – Leflunomid/Teriflunomid) and in utility model cases (BGH, judgment of June 7, 2006 – X ZR 105/04 – Luftabscheider für Milchsammelanlage).

In its recent leading decision ”Fensterflügel“, the BGH generalized these principles to the effect that the same legal consequence can also occur if the patent proprietor makes a comparable declaration in a different context and the recipient of the declaration takes actions in justified reliance on this declaration, which the patent proprietor later attacks with an infringement action (judgment of November 3, 2020 – X ZR 85/19). Although the BGH refers to declarations “in a different context” in general, one should not conclude from this that declarations of a patent proprietor should have an effect “erga omnes”. It is also to be noted that such a declaration should be “comparable” to the circumstances contemplated in the previous case law, thus the decision appears to intend to refer to declarations in other “inter partes” situations.

In the case in dispute, the statement of a patent proprietor was at issue, with which he had defended against a negative declaratory action. The statement was to the effect that ”at present“ no claims were being considered with regard to the embodiment now in dispute. However, when the patent proprietor then attacked this embodiment in a subsequent action, the former opponent countered the earlier statement. The BGH has now followed this insofar as the invocation of such a declaration is possible in principle. In the case in dispute, however, it did not allow the declaration with the qualification ”at present“ to suffice in terms of content.

Since the BGH did not elaborate on whether the declaration of the patent proprietor is made within formal legal proceedings, the decision means for practice that patent proprietors also have to pay attention to how they express themselves on concrete infringement issues outside such proceedings.