To describe the epidemiology of infection with hepatitis C virus (HCV), hepatitis B virus (HBV) and human immunodeficiency virus (HIV) among injecting drug users (IDUs) in Australia, in relation to the potential for further spread of HIV in IDUs, a cross-sectional analysis was performed on data from a sample of injecting drug users, correlating markers of exposure to blood-borne viruses with sex, age, sexual orientation, primary current drug injected and duration of injecting in rural and metropolitan Victoria, Australia. The subjects were currently active IDUs from a wide spectrum of age, sex, sexual orientation, geographical location and social background, contacted and recruited through their social networks and from community agencies and prisons by trained peer workers who interviewed and collected blood from them in the field. Sera were tested for antibody to HIV, HCV and hepatitis B core antigen (HBcAg), for hepatitis B surface antigen (HBsAg), and for HCV RNA using reverse transcription and polymerase chain reaction (RT-PCR). At entry to the study, 4.5% (14/311) had antibody to HIV, 47% (146/308) to HBcAg and 68% (206/303) to HCV. Prevalence of HBsAg was 1.8% overall (5/282), and 50% (84/168) were positive for HCV RNA. By multivariate analysis, HIV seropositivity was strongly associated with a history of homosexual contact in males and with exposure to HBV but not to HCV. Those who reported their current primary injected drug to be amphetamines were at greater and continuing risk of HIV infection than were current heroin injectors, while the reverse applied for HCV.(ABSTRACT TRUNCATED AT 250 WORDS)