On December 22, 2006 the Dutch Supreme Court ruled in the case Dijkstra vs. Saier that the patent prosecution file history may be invoked to prevent the patentee disclaiming an aspect of his patent monopoly in order to get a patent granted, but then reclaiming it during infringement proceedings. In Anglo American practice this is known as file wrapper estoppel, “amendment estoppel”, or “argument estoppel”. This prevents a proprietor who has narrowed his claims during prosecution in order to meet requirements of patentability from then asserting the patent against any equivalent technologies falling within the scope of a surrendered territory. The latter type of estoppel (“argument estoppel”), is similar as amendment estoppel, in that where a patentee has made statements to the Patent Office (or in the Dijksta vs. Saier case) during prosecution – namely during the opposition proceedings before the European Patent Office – with regard to the scope of a claim, he cannot subsequently assert the patent against equivalent technology in contradiction of that assertion.The Court thus affirmed its findings in the first case on the use of prosecution file history in a patent infringement case, Ciba-Geigy vs. Oté Optics (NJ 1995/391). In that case the Dutch Supreme Court ruled that the scope of protection of a patent has to be ascertained by considering four factors: (1) in interpreting the terms of the claims, the Court is to determine the essence of the invention; in other words, consider the inventive concept behind the wording of the claims; (2) this interpretation then needs to be corrected to give a reasonable degree of certainty for third parties, which may sometimes justify a restricted, literal interpretation of the wording of the claims; (3) the skilled person may – with restraint – use the prosecution history file for the purpose of claim interpretation; (4) and all other circumstances of the case are to be taken into account, including the possible ‘breakthrough’ nature of an invention (justifying a broader scope). When considering factors (2) and (3), poor drafting of the patent may be construed to be patentee’s disadvantage. In subsequent decisions (including Impro vs. Liko, rendered three weeks after Amgen in the UK), the Netherlands Supreme Court has expressly confirmed this approach. The approach certainly appears to be more in line with the EPC requirements, although there is a lingering feeling amongst practitioners that, underlying the approach, the Supreme Court still continues to embrace its ‘essence of the invention’ approach. In Van Bentum vs. Kool (HR 29 maart 2002, NJ 2002/530 m nt. ChG), a clear case of non-literal infringement, the Supreme Court has clarified the approach in Ciba-Geigy vs. Oté Optics such that the skilled person is only to assume that the patentee has surrendered part of the protection (e.g. beyond the literal wording of the claims but within the full extent of the invention) if there is ‘proper ground’ for the skilled person to do so. Such “proper ground” can for example be found in the patent description or the prosecution history file. This was confirmed in the case Impro vs. Liko, HR 12 November 2004, NJ 2004, 674).