One feature of the European patent system that is heavily criticized nowadays is related to its complex fragmentation and the induced cost burden for applicants. Once a patent is granted by the EPO, the assignee must validate (and often translate) it and pay the renewal fees to keep it in force in each country in which the applicant seeks protection. The objective of this paper is to assess to what extent validation and renewal fees as well as translation costs affect the validation behavior of applicants. We rely on a gravity model that aims at explaining patent flows between inventor and target countries within the European patent system. The results show that the size of countries, their wealth and the distance between their capital citi...