Sexual behaviors of a group of 313 intravenous drug users (IVDUs) (225 men, 88 women) were assessed by a structured interview at the start of an AIDS prevention project. Although the majority were celibate or monogamous during the prior year (men 52.5%, women 64.3%), many IVDUs had multiple sexual partners in that time, including 19.6% of men and 7.2% of women reporting five or more. Married male and female IVDUs were more likely to have multiple sexual partners than married people in general population samples. Only 38.5% of male and 35.4% of female IVDUs ever used condoms during the previous 5 years. Percentage of sexual encounters in which condoms were used correlated significantly with the number of partners for women (r = 0.37, p less than 0.01), but not for men (r = 0.03, ns). Use of intravenous drugs in the prior year, sharing of needles, use of stimulants, exchange of sex for money or drugs, and combining sexual events with drug use were all associated with greater numbers of sexual partners. A subset of male IVDUs engages in the dual-risk behaviors of needle sharing and unprotected intercourse with multiple partners. This group could act as a highly efficient vector for human immunodeficiency virus transmission and should become a focus of intensive preventive intervention.