We measure how patent publication affects innovation by exploiting the American Inventor’s Protection Act of 1999 (AIPA), which accelerated public disclosure of US patents by about 1.5 years. We obtain causal estimates by comparing US patents subject to AIPA with “twin” European patents that were not. Post-AIPA, US patents receive more and faster follow-on citations, indicating greater technology diffusion. Technological overlap increases between distant but related patents and decreases between highly similar patents, and patent applications are less likely to be abandoned, suggesting less duplicative R&D. Publicly listed firms exposed to 1 standard deviation longer patent grant delays increased R&D by 4% post-AIPA.