To determine the prevalence of antibodies to Hepatitis A virus (anti-HAV) among drug addicts, sera collected in a Swedish city during a ten-year period from 234 drug addicts with acute hepatitis B were tested for anti-HAV. The results were compared with the normal population, where only 3.8% of those born after 1950 were anti-HAV-positive. In individuals born between 1941 and 1965, 8.2% in the normal population and 30.2% of the drug addicts were anti-HAV-positive (p less than 0.001). The level of immunity to hepatitis A among drug addicts ranged from 7.7% to 60% during the ten-year period. Low levels of immunity were seen in the years preceeding outbreaks of hepatitis A among drug addicts. These outbreaks occurred in a cyclic pattern. Higher levels of immunity were seen after each outbreak.