To determine whether human immunodeficiency virus (HIV) type 1 infection among unprotected receptive anal partners of HIV type 1-infected men varies by the immunologic status of the HIV type 1-infected index case, 187 sexual partners of 164 HIV type 1-infected index subjects were enrolled at a community health center in Boston, Massachusetts, from 1985-1990. All subjects were interviewed regarding their sexual practices and tested for HIV type 1. Fifty-seven of the 187 sexual partners were infected with HIV type 1. The strongest risk factor for HIV type 1 infection among these partners was unprotected receptive anal intercourse with a known HIV type 1-infected index subject (odds ratio (OR) = 7.2, 95% confidence interval (CI) 3.1-16.3). The risk of unprotected receptive anal intercourse was highest among partners of HIV-infected index subjects who had a T lymphocyte subset ratio of 0.50 or less (OR = 11.4, 95% CI 3.0-43.5) compared with partners of HIV type 1-infected index subjects with a T cell ratio of greater than 0.50 (OR = 5.3, 95% CI 1.9-15.2). After adjustment for confounding, the risk of HIV type 1 infection remained substantially higher among sexual partners who had had unprotected receptive anal intercourse with infected index subjects with a T lymphocyte subset ratio of less than or equal to 0.50 (OR = 7.0, 95% CI 1.8-28.0) compared with partners who had had unprotected receptive anal intercourse with infected index subjects with a T cell ratio of greater than 0.50 (OR = 3.3, 95% CI 1.1-10.0). It would appear that the risk of HIV type 1 infection from unprotected receptive anal intercourse increases as the immunologic status of the HIV type 1-infected insertive anal partner decreases.