HIV infection and AIDS have posed unprecedented problems for prison authorities around the world. The first known HIV-infected prisoner in an Irish prison was identified in 1985. Between January 1987 and January 1991, 168 known HIV-infected prisoners were incarcerated in Mountjoy prison in Dublin, a figure representing 16.6% of the total HIV-infected population in the Republic of Ireland over the same period. Of these, 84% attended St James's Hospital. They displayed considerable morbidity from HIV-related disease, mainly respiratory tract infection, although additional morbidity was directly due to intravenous drug use. A questionnaire was used to gather information on a representative sample of inmates. Of the 34 individuals studied, 29 were imprisoned for drug-related crimes, and 32 reported parenteral drug use within the prison. This study highlighted a number of important points relating to the burden of HIV disease on an Irish prison population.