A prolonged outbreak of hepatitis A infection amongst drug users in Suffolk prompted a study of the natural immunity against hepatitis A in this population, and a retrospective analysis of the relationship between specific drug-taking behaviours and the risk of hepatitis A infection. Prior to the outbreak, age-specific seroprevalence of hepatitis A IgG in drug users was similar to that amongst blood donors in the region. Of those without effective immunity, intravenous drug users, multiple drug users and those injecting frequently were more likely to have developed hepatitis. The reported frequency of equipment sharing and the number of injecting partners were not related to the risk of infection. The potential for blood-to-blood, and a suggested faecal-blood transmission were considered to be important in propagating the outbreak in this population. We suggest that a single dose of hepatitis A vaccine administered opportunistically should be used in outbreaks involving drug users.