In a voluntary anonymous HIV and hepatitis C serology screen in a Canadian male medium security federal penitentiary, 68% of 520 prisoners volunteered a blood sample and 99% of those giving a blood sample completed a risk behaviour questionnaire which was linked numerically to the blood sample. Compared to previous screenings for HIV (4 years earlier), and hepatitis C (3 years earlier) in the same institution, HIV seroprevalence had risen from 1% to 2% and hepatitis C seroprevalence from 28% to 33%. The overwhelming risk association for hepatitis C was with drug use outside prison, although there was a small group of men who had only ever injected drugs inside prison, over half of whom had been infected with hepatitis C. The proportion of prisoners who had injected drugs in prison rose from 12% in 1995 to 24% in 1998. The proportion of surveyed individuals sharing injection equipment at some time in prison was 19%, and while HIV rates in the prison are currently low, HIV prevalence amongst Canadian street i.v. drug users is rising rapidly, underlining the need for urgent preventative measures in prisons.