In February 1949, the Air Force replaced Project Sign with Project Grudge, an investigation explicitly tasked with debunking UFO reports and reducing public alarm rather than pursuing the extraterrestrial hypothesis advanced by Sign's analysts.

Project Grudge published its final report in December 1949, explaining all sightings as misidentifications, mass hysteria, or hoaxes. Astronomer J. Allen Hynek later criticized Grudge as 'less science and more of a public relations campaign.' Project Blue Book succeeded it in 1952.