Compulsory invention secrecy is a policy tool used primarily for preventing technology leaks to foreign competitors. This column exploits a natural historical experiment to study the impact of compulsory secrecy on the wider innovation system. During WWII, the US patent office issued secrecy orders on more than 11,000 patent applications, halting their examination and prohibiting disclosure. The effects of this policy – which prompted incumbent firms to shift the direction of their research – persisted through 1960, restricting commercialisation and impeding follow-on innovation while successfully keeping sensitive technology out of public view.