Since 1981, over 456,000 AIDS cases have been reported to the Centers for Disease Control. Mean time of onset between HIV infection and AIDS onset exceeds 7 years. Observed prevalence of infection remains highest among those groups accounting for the majority of AIDS cases: homosexual and bisexual men, intravenous drug abusers, and sexual partners of these high-risk groups. Other lesser-risk groups include hemophiliacs, those who received contaminated blood or blood products, and children born to infected mothers. While there is evidence that new infections continue to occur among risk groups, blood donors, and military personnel, rates of infection may have declined somewhat since the early 1980's as the result of behavior change, education, and serologic screening. Based on estimates of the size of at-risk populations and estimated seroprevalence values for those groups, it is estimated that between 1 and 1.5 million Americans are infected with HIV.