What proportion of patent applications filed at the U.S. Patent and Trademark Office (USPTO) are eventually granted? Many experts have suggested that the USPTO approves nearly all applications, blaming this apparent leniency for many problems with the U.S. patent system. To test this assumption, we follow the prosecution histories of 2.15 million U.S. patent applications from 1996 to mid-2013. We find that only 55.8% of the applications emerged as patents without using continuation procedures to create related applications. The allowance rate has decreased substantially over time, particularly for applications in the “Drugs and Medical Instruments” and “Computers and Communications” fields. Furthermore, applications filed by small firms were less likely to emerge as patents than those filed by large firms. We discuss the implications of our findings for inventors, policymakers, and legal scholars.