The prevalence rates of serum hepatitis B surface antigen (HBsAg) and antibody to HBsAg (anti-HBs) were 5.6% and 34% in 144 homosexual men in Seattle. Prevalence rates were only 0.9% and 3.6% in 111 heterosexual male venereal disease clinic patients with nongonococcal urethritis, and also 0.9% and 3.6% in 111 healthy men undergoing routine physical examinations. Thus, previous exposure to hepatitis B virus (HBV) was estimated to be 8.8 times greater for homosexual men than for heterosexual men. Four of four HBsAg positive sera from homosexual men were subtyped as "ad," whereas subtype "ay" is preponderant in intravenous drug abusers. Future public health measures to control HBV infection should address the prevention of sexually transmitted HBV infection among homosexual men.