This paper reports on the British findings from a cross-national study of HIV prevalence and HIV risk behaviour among 1,037 injecting drug users (IDUs) recruited from a variety of treatment- and community-based settings during 1990. Confirmed HIV saliva test results show 12.8% (63) of London respondents and 1.8% (8) of Glasgow respondents to be HIV antibody positive. Among London respondents, a higher rate of prevalence was found in those with no experience of drug treatment. A greater proportion of Glasgow respondents (68%) than London respondents (47%) reported sharing used injecting equipment in the 6 months prior to interview. The majority (88% in both cities) attempted cleaning borrowed equipment, although less than a third (31% in London and 30% in Glasgow) usually used bleach. The majority of respondents (71% in London and 82% in Glasgow) were sexually active with partners of the opposite sex in the last 6 months, and respondents had a mean number of 2.4 non-commercial sexual partners in London and 2.1 in Glasgow. Levels of reported condom use were comparable with reports in the heterosexual population as a whole, with 70% of London respondents and 75% of Glasgow respondents never using condoms with primary partners, and 34% of London and 52% of Glasgow respondents never using condoms with casual partners. Half (48%) of London respondents and 42% of Glasgow respondents reported sexual intercourse with non-injecting private sexual partners, while 14% of female respondents in London and 22% in Glasgow had engaged in prostitution. Levels of risk-taking in each of the two cities indicate the potential for further transmission of HIV among drug injectors, and their sexual and sharing partners.