The subjects ranged in age from 21 to 61 and had been involved in a sexual relationship with an index AIDS case from 2 to 221 months. Of the 117 subjects, 41 percent reported sexual relations with the index case at the time of his AIDS diagnosis, 33 percent had their last sexual contact with the index case from one to 6 months before the diagnosis and the remaining 26 percent from 7 to 24 months before diagnosis. Receptive anal intercourse with the index AIDS case and number of different sexual partners with whom subjects were anally receptive were both risk factors. Controlling for the number of partners with whom subjects were anally receptive, findings indicate that the odds ratio of receptive anal intercourse with the case was infinite (95% confidence intervals, 3.3) if sexual contact continued up to or beyond the time of diagnosis, while the odds ratio was 1.0 (95% CIO.3-3.2) if contact ceased before the case's AIDS diagnosis. Risk was not associated with the duration or frequency of contact. The data suggest that the potential for sexual transmission from an HIV-infected person may be greater close on or after the onset of disease. (Author abstract modified).