From the page: "Since the inception of the Brady Act, over 118 million applications for firearm transfers or permits were subject to background checks. About 2.1 million applications, or 1.8%, were denied.
In 2010, 1.5% of the 10.4 million applications for firearm transfers or permits were denied by the FBI (approximately 73,000) or by state and local agencies (approximately 80,000).
Among the 21 state agencies that reported reasons for denial, a felony conviction or indictment was the most common reason to deny an application in 2010 (31%). A state law prohibition (16%) was the second most common reason (excluding other prohibitions)."