The importance of sharing injecting equipment in the transmission dynamics of HIV is well established. Comparatively less is known about the sexual behaviour of drug injectors and the risks posed to themselves and their sexual partners through the sexual transmission of HIV. Findings are reported from survey-based interviews undertaken in 1991 which investigated the sexual behaviour of 516 drug injectors, both in and out of treatment in London. The majority of respondents (80%) were sexually active in the 6 months preceding interview. During this time, respondents had a mean of 2.1 non-commercial opposite sex partners. Most (66%) had vaginal intercourse at least once a week, although 68% never used condoms with primary partners and 34% never used condoms with casual partners. Those having sexual intercourse most often were less likely to use condoms. Many had non-injecting sexual partners, and 62% of respondents' primary and casual partners did not inject drugs. Confirmed saliva HIV test results show 10% of respondents to be antibody positive, with a higher rate of prevalence (14% positive) among those with no experience of treatment. This group were also more likely to report casual sexual intercourse. The average rate of partner change, the high proportion of drug injectors with non-injecting partners and the infrequency of condom use within primary and causal relationships indicates the potential for HIV transmission between injectors and their non-injecting sexual partners. The paper concludes by emphasizing the importance of outreach and community-based intervention in safer sex health promotion.