This paper examines the sexual behaviors of 255 intravenous drug users (IVDUs) to assess the potential for the sexual transmission of HIV and to examine differences in sexual behaviors between in- and out-of-treatment IVDUs. In-treatment subjects (N = 152) were a random sample of clients at a large, publicly funded methadone maintenance program. Out-of-treatment subjects (N = 103) were recruited through a chain referral technique, using the in-treatment subjects. Forty-five percent of the study subjects reported multiple partners and 32% reported exchanging sex for money or drugs in the preceding 6 months. Fifty-three percent reported no use of condoms. After controlling for demographic differences between the in- and out-of-treatment groups, out-of-treatment IVDUs reported significantly more partners than in-treatment IVDUs (4.6 vs 2.3, significant t < 0.01), and more often had exchanged sex for money or drugs (44 vs 26%, relative odds 1.8, p < .05). In- and out-of-treatment subjects did not differ with respect to condom use. We conclude that IVDUs both in- and out-of-treatment continue to be at risk of contracting and spreading HIV infection through sexual behaviors, and that being in drug treatment is associated with a lower incidence of high risk sexual behaviors.