During routine testing of civilian applicants for U.S. military service, the overall seroprevalence of antibodies to HIV-1 in 2011 was the second lowest of any year since 1990. Among members of the active components of the U.S. Army, HIV-1 seroprevalences were higher during 2008 to 2011 than in recent prior years. Among members of the active components of the U.S. Air Force, Navy and Marine Corps, the Marine Corps Reserve, and the Army National Guard, HIV-1 seroprevalences have slightly declined or remained relatively stable for at least ten years. In the reserve components of most service branches, it is difficult to discern long-term trends because of instability of seroprevalences in the relatively small numbers of reserve component members tested each year. Monitoring of HIV-1 seroprevalences can help target and focus prevention initiatives. The recent repeal of the Don't Ask Don't Tell policy has created opportunities for prevention messages targeted to men who have sex with men.