To evaluate behavioral and immunologic factors related to transmission of human immunodeficiency virus (HIV) by homosexual intercourse, we studied a population of 329 homosexual/bisexual men (155 partner-pairs) seen in a community health center and medical outpatient clinic. Logistic regression analysis showed that behavioral risk factors for infection in the 130 HIV-infected men included: receptive anal intercourse (OR 4.6, 95% CI-1.8, 12.1); receptive fisting (OR 2.5, CI-1.1, 7.0); nitrite use (OR 2.3, CI-1.2, 4.6); history of gonorrhea or syphilis (OR 2.3, CI-1.4, 3.9); and history of sexual contact with men from areas with many AIDS cases (OR 1.9, CI-1.0, 3.5). Comparing seven men who were probable transmitters of HIV and 11 men who had not transmitted HIV to their uninfected partners despite unprotected insertive anal intercourse, we found no differences in HIV isolation from peripheral blood mononuclear cells, circulating HIV antigen detection, or presence of neutralizing antibody to HIV. Helper T-cell numbers were not significantly different between the two groups, but transmitters had more suppressor T-cells than did nontransmitters.