Inventions of foreign origin are about ten percentage points less likely to be granted a U.S. patent than domestic inventions. An empirical analysis of 1.5 million U.S. patent applications identifies three systematic differences between foreign and domestic patent applications that partly explain this bias. They include differences in patent agents, the financial resources of the applicants, and the level of effort that applicants put into the prosecution process. We find no evidence of disparate treatment (‘intentional discrimination’) of foreigners. Instead, our evidence points to a disparate impact (‘unintentional discrimination’) of the U.S. patent system on foreign inventors. Our results suggest unequal access to the patent system for foreigners compared to locals (but also for small U.S. firms). Giving examiners the power of (truly) rejecting a patent application may be one solution to level the playing field between foreigners and locals, but also between large and small firms.