Forty-nine percent of homosexual/bisexual men were positive for antibody to the human immunodeficiency virus (HIV) in a population-based probability sample of 1034 single men recruited from San Francisco. All heterosexual men were negative. Among seropositive men, marked lymphadenopathy was present in 29%, and 16% had at least two other symptoms or signs suggestive of HIV infection. However, lymphadenopathy alone failed to indicate severity of immune impairment. The occurrence of two or more clinical signs and symptoms, except for marked lymphadenopathy, correlated with HIV infection, diminished skin test reactivity, and reduction in Leu 3a T cells. Twenty-nine percent of seropositive men had fewer than 400 absolute Leu 3a T helper cells per microliter (less than 0.4 X 10(9)/L). Seronegative homosexual/bisexual men did not differ from heterosexual men in any clinical or laboratory variables except for increased numbers of suppressor Leu 2a T suppressor cells per microliter.