In order to assess modes of human immunodeficiency virus (HIV) transmission from heterosexual intravenous drug users (IVDUs) to their partners, condom use and sexual habits with both steady and occasional partners were investigated. A total of 349 heterosexual IVDUs (247 men and 102 women) who ignored, at the time of interview, their HIV serostatus were interviewed. Respondents were asked for information on condom use and sexual habits for the three year period prior to the interview. Nearly 40% of IVDUs reported sexual intercourse with both steady partners and occasional partners. Fifty-four percent of their steady partners and 48% of their occasional partners were individuals who did not belong to groups at risk for HIV infection. Anal intercourse with steady partners was reported by 29% of IVDUs and 24% of IVDUs with occasional partners. Condom use during vaginal intercourse was seldom reported: 83% of IVDUs never used a condom with steady partners and 75% did not use one with occasional partners. IVDUs who were 1) unmarried, 2) enrolled in the study after 1986, 3) partners of not at-risk individuals, 4) partners of a foreigner and, 5) aware of their partners HIV seropositivity showed significantly higher, albeit still low, frequencies of condom use with steady partners. Conversely, all these factors seemed to have little impact on condom use with occasional partners. Condom use and sexual habits were similarly reported by HIV-positive and HIV-negative IVDUs. The present study shows that high-risk sexual behaviours among IVDUs are very widespread and it stresses the need for intensive counselling to promote condom use among IVDUs.