To assess prevalence of exposure to hepatitis A virus (HAV) among injecting drug users (IDUs) and prison entrants in Victoria, and to compare this with prevalence of HAV among a reference population of blood donors, sera stored from two previous studies and from randomly selected blood donors were tested for total antibody to HAV. The first study was a longitudinal study of field-recruited IDUs from 1990 to 1992 and the second was a study of all prison entrants in 1991-92 (both studies were carried out in Victoria); blood donors were from the Australian Red Cross Blood Bank Victoria in 1995. Forty-five per cent of 2175 prison entrants and 51% of 293 IDUs were seropositive for HAV, compared with 30% of 2995 blood donors. When standardized for age against the blood donors, HAV seropositivity in IDUs was 44% and in prison entrants 60%. The strongest association of HAV seropositivity among the IDUs on multivariate analysis was a history of imprisonment. There are high rates of exposure to HAV among prison entrants, whether with a history of IDU or not, and among IDUs who have a prison history. The role of sharing contaminated injecting equipment in transmission of HAV seems to be less important than institutionalization per se. With adequate resourcing, both populations are appropriate targets for HAV vaccination, especially in a context of continuing decline of transmission of HAV in the general community.