This paper studies the effects of the USPTO's patent secrecy program in World War II, under which approximately 11,200 U.S. patent applications were issued secrecy orders that halted examination and prohibited inventors from disclosing their inventions or filing in foreign countries in the interests of national security. Secrecy orders were issued most heavily in areas important to the war effort—including radar, electronics, and synthetic materials—and nearly all rescinded en masse at the end of the war. I find that compulsory invention secrecy was effective at keeping affected technology out of the public domain, but it appears to have reduced and delayed follow-on invention, reduced entry into patenting, and restricted commercialization. The results shed light on the consequences of invention secrecy, which is widely used by inventors to protect and appropriate the returns to innovation and yield lessons for ongoing policy debates over potential measures to protect U.S. invention against the growing incidence of foreign IP theft today.