A pilot programme of hepatitis B vaccination was set up in three prisons to assess the feasibility and results of this method of reaching a high-risk population. Hepatitis B vaccine was offered to all inmates who lacked serological markers for hepatitis B virus. The antibody response was assessed in those who received two or three doses of vaccine. Candidates for vaccination were 41% of 1755 imprisoned men (20% of intravenous drug users (IVDU) and 63% of non-IVDU), but complete vaccination could be given to only 33% of candidates. A further 29% received two doses. Seroconversion to anti-HBs (titres > 10 IU l-1) occurred in 33% of vaccinees after two doses and in 76% after three doses. The overall rate of susceptible inmates who became protected for hepatitis B was 34%. The seroconversion rate was higher when the interval between the first two doses was shorter than 3 weeks (91%), than in cases with an interval of 3-6 weeks (79%) or longer than 6 weeks (33%). Age greater than 35 years and history of IVDU were associated with a lower response to the vaccine, while anti-HIV seropositivity did not influence the response. In conclusion, vaccination of prisoners susceptible to HBV may achieve protection in at least a third of cases. Shortening intervals between the priming doses of vaccine may improve compliance and increases the response.