The sexual behaviour of 142 clients of syringe-exchange schemes was measured using a questionnaire interview based survey method. Two to four months later the questionnaire was repeated to provide measures of behavioural change. The majority of these clients were sexually active, 77% having one or more sexual partners in the 3 months prior to the first interview. Many of these clients (46% of those sexually active) had sexual partners who did not inject drugs. There is evidence that this group modified their sexual behaviour towards reducing their own risk of HIV infection, with more having no sexual partners (from 23% to 31%), a reduction in those having multiple partners (from 26% to 21%) and a slight increase in those with regular partners (from 49% to 52%). Not all clients in this group reduced their risk of infection by modifying their sexual behaviour. Some clients continued to engage in high risk sexual behaviour, having multiple partners (21%) and not using condoms (79%). Whilst overall there is evidence of changes in the sexual behaviour of the clients towards a reduced risk of infection, we have identified an associated increase in risk of transmission from them to their sexual partners. Drug injectors who continued to have sexual partners were more likely to have sexual partners who did not inject drugs.