This study investigates the extent to which heroin users are exposed to multiple forms of infection risk. Structured interviews were administered to a prospective network sample of 408 heroin users. Subjects were contracted in south London in a wide range of social settings by specially recruited privileged access interviewers. Most heroin users (74.5%) had been exposed to more than one infection risk factor and more than half of the sample had been exposed to three or more risk factors. HIV serostatus was primarily related to men having sex with men. Hepatitis B seropositive status was primarily related to the number of years injecting drugs. At this stage of the HIV epidemic in London, HIV infection among heroin users may be related more to homosexual risk behaviour than drug risk factors. Heroin injectors were at greater risk of infection than heroin chasers both through their sexual behaviour as well as through their injecting practices. Heroin users who refused to give a saliva sample for analysis were found to be more likely to engage in several health risk behaviours than those who provided samples. This finding has important methodological implications for seroprevalence surveys. Other implications of the results for prevention programmes aimed at health risk behaviours of heroin users are also discussed.