Medical, sexual, and drug-use histories were obtained for all subjects (300 in the treatment group and 138 in the out-of-treatment group), and T-cell subsets were performed and sera were screened for the presence of human immunodeficiency virus-type one (HIV-1) antibodies. An overall HIV-1 seroprevalence rate of 9 percent was found among the 401 IVDU's for whom seriological data were obtained. This contrasts with the estimated seroprevalence of 40-50 percent for San Francisco gay males and the estimated 50 percent among IVDU's in New York City. The seroprevalence for the out-of-treatment IVDU's tested was 16 percent compared with 7 percent for the treatment group. This was due partly to the higher percentage of males in the out-of-treatment group who had histories of receptive anal intercourse. Ninety-five percent of the IVDU's surveyed reported sharing needles. Most reported that they cleaned needles in some way between sharing partners. Overall, findings suggest that AIDS prevention intervention with IVDU's in treatment will not be sufficient to impact the spread of the virus in the IVDU population. 18 references.