Prisoners in eight of the 135 prisons in England and Wales were surveyed in 1997 and 1998 to study the prevalence of and risk factors for transmission of bloodborne viruses in prison. Subjects voluntarily completed a risk factor questionnaire and provided oral fluid specimens for unlinked anonymous testing for the presence of antibodies to HIV, hepatitis C virus (HCV), and the core antigen of hepatitis B virus (HBc). Almost 8% (4778) of the total of 60,561 prisoners were eligible and four fifths (3942) of those eligible took part. Among all those tested (3930) 0.4% (14) were positive for anti-HIV, 8% (308) for anti-HBc, and 7% (293) for anti-HCV (the anti-HBc and anti-HCV prevalences were not adjusted for assay sensitivities of 82% and 80%, respectively). Twenty-four per cent (777/3176) of adult prisoners reported ever having injected drugs, 30% of whom (224/747) reported having injected in prison. Three quarters of those who injected in prison (167/224) shared needles or syringes. Among adult injecting drug users, 0.5% (4/775) had anti-HIV, 31% (240/775) anti-HCV, and 20% (158/775) anti-HBc. The presence of anti-HCV and anti-HBc was associated with injecting inside prison and number of previous times in prison. The results suggest that hepatitis viruses are probably being transmitted in prisons through sharing non-sterile injecting equipment and that a risk of HIV transmission exists. Harm minimisation measures for the 6% of prisoners who continue to inject while in prison should be strengthened.