Self-reported hepatitis B virus (HBV), hepatitis C virus (HCV), and HIV infection status was compared with the results of oral fluid assays of antibodies to these viruses in prisoners from nine of the 15 prisons in the Republic of Ireland. A total of 1205 out of 1366 prisoners completed a confidential questionnaire and 1193 provided analysable oral fluid specimens for testing for antibodies to HBV core antigen (anti-HBc), HCV (anti-HCV), and HIV (anti-HIV). The self-reported prevalence of hepatitis infection (hepatitis B: 5%; hepatitis C: 19%) was lower than that derived from oral fluid assays (anti-HBc: 9%; anti-HCV: 37%). The self-reported prevalence of HIV infection was similar to that found by oral fluid assay (2%). Many discrepancies were found between self-reported results and the results of oral fluid assays. Of those who reported being positive for HBV, HCV, or HIV, 48%, 5%, and 58%, respectively, tested negative on the oral fluid assay. Of those who reported a previous negative test result for HBV, HCV, or HIV, 10%, 37%, and 2%, respectively, had positive oral fluid assays. Self-reports of hepatitis and HIV infection status are unreliable and should not be used as a basis for planning preventive and treatment services for prisoners. All prisoners should have the opportunity to be tested for HBV, HCV, and HIV infection.