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The paper investigates whether patent fees are an effective mechanism to deter the filing of low‐quality patent applications. The study analyzes the effect on patent quality of the Patent Law Amendment Act of 1982, which resulted in a substantial increase in patenting fees at the U.S. Patent and Trademark Office. Results from a series of difference‐in‐differences regressions suggest that the increase in fees led to a weeding out of low‐quality patents. About 10% of patents in the lowest quality decile were filtered out, with the effect concentrated in the patents of firms whose overall patent portfolio was medium to large (more than 20 patents). The study has strong policy implications in the current context of concerns about declines in patent quality.