Prisoners and attending staff from six houses of detention were screened for serum HBsAg, anti HBs, antiHBc and transaminases. Both prisoners and warders showed an increased prevalence of HBV serum markers with respect to age- and sex-matched general population control groups. The HBsAg carrier rate was 6.7% in prisoners and 6.6% in staff. Sixty-five per cent of the HBsAg-positive subjects were antiHBe-positive. When the blood samples were taken, all the HBsAg-positive subjects were asymptomatic; transaminases were normal in 80% and only slightly elevated in 20%. Illicit drug abuse was found to be a relevant risk factor for HBV infection among prisoners under 35 yearts of age, but not in the older group, whereas no correlation emerged between presence of HBV serum markers and tattoos or admitted homosexuality. A high prevalance of HBV serum markers as also found among young warders who had been in service for a short time: most of them, however, come from areas of Italy with a particularly elevated HBV circulation. Although the results suggest that many of the HBV infections are not recently acquired within the institution, this survey confirms that prisons should be regarded as high risk areas for HBV infection, both for prisoners and warders.