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In recent years, patent protection has extended into new areas, giving rise to serious concern about the lack of clear guidelines for patentability. We analyze the effect of introducing a patent opposition process that would allow patent validity to be challenged directly after a patent is granted. In many cases, such a system would avoid costly litigation at a later date. In other cases, the opposition process would increase the cost of conflict resolution, but would also reward holders of valid patents and limit the rewards to invalid patents. Our analysis suggests significant positive welfare gains from the introduction of a patent opposition process.