OBJECTIVES: To assess spread of bloodborne viruses among prison entrants in Victoria, Australia. DESIGN: Voluntary confidential testing of all prison entrants for markers of exposure to bloodborne viruses with collection of minimal data on demography and risk factors over 12 months. SETTING: Her Majesty’s Prisons, Pentridge and Fairlea, Victoria, Australia. SUBJECTS: 3429 male and 198 female prison entrants (> 99% of all prison entrants); 344 entered prison and were tested more than once. MAIN OUTCOME MEASURES: Prevalence and incidence of antibodies to HIV, hepatitis B, and hepatitis C viruses, and minimal data on risk factors. RESULTS: 1562 (46%) gave a history of use of injected drugs, 1171 (33%) had antibody to hepatitis B core antigen, 1418 (39%) were anti-hepatitis C positive including 914 (64%) of the men who injected drugs, 91 (2.5%) were positive for hepatitis B surface antigen, and 17 (0.47%) were positive for antibody to HIV. Incidence rates for infection with hepatitis B and C virus were 12.6 and 18.3 per 100 person years, respectively; in men who injected drugs and were aged less than 30 years (29% of all prison entrants) these were 21 and 41 per 100 person years. Seroconversion to hepatitis B or C was associated with young age and shorter stay in prison. Only 5% of those who were not immune to hepatitis B reported hepatitis B immunisation. CONCLUSIONS: Hepatitis B and C are spreading rapidly through some populations of injecting drug users in Victoria, particularly among men aged less than 30 years at risk of imprisonment in whom rates of spread are extreme; this group constitutes a sizeable at risk population for spread of HIV. This spread is occurring in a context of integrated harm reduction measures outside prisons for prevention of viral spread but few programmes within or on transition from prisons; it poses an urgent challenge to these programmes.