The paper investigates whether patent fees are an effective mechanism to deter the filing of low-quality patent applications. The study analyzes the effect 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, on patent quality. 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 16-17 per cent of patents in the lowest quality decile were filtered out. The figure reaches 24-30 per cent for patents in the lowest quality quintile. However, the fee elasticity of quality decreased with the size of the patent portfolio held by applicants. The study has strong policy implications in the current context of concerns about declines in patent quality and the financial vulnerability of patent offices.Also published as NBER Working PaperNo. w20785.