To assess the incidence of heterosexual HIV-1 transmission from seropositive methadone-maintained injecting drug users (IDU) to their seronegative, non-IDU steady sex partners. A prospective, longitudinal study. HIV-1-seropositive IDU and their HIV-1-seronegative, non-IDU steady sex partners were recruited from six methadone-maintenance clinics in the New York City boroughs of Brooklyn and Manhattan. Between December 1988 and October 1991, 60 research participants were recruited [30 methadone-maintained, HIV-1-seropositive IDU (index subjects) and their non-IDU HIV-1-seronegative steady partners]. Baseline and follow-up (every 3-4 months) data included a structured questionnaire and blood testing for HIV infection and CD4+ cell subsets. Demographic characteristics of the 30 steady sex partners were evaluated as follows: sex (22 women, eight men); age (mean, 32 years; range, 21-55 years) and race (African-American, 13; Hispanic, 13; other, 4). The average duration of relationship between index subject and partner at study entry was 7.5 years (range, 4 months to 19.4 years). At entry, 27 of the 30 partners reported no or intermittent use of condoms with the index subject during a typical 12-month period. Follow-up data (median follow-up, 298 days; range, 85-992 days) were available for 22 partners. Of these, 15 reported no or intermittent condom use. For a total study observation time of 23 person-years, there were no seroconversions. The upper 95% confidence interval for the observed seroconversion rate of 0% is 13%. These results suggest that the likelihood of heterosexual HIV-1 transmission from seropositive methadone-maintained IDU to their seronegative, non-IDU steady sex partners is low.