Between January 1982 and December 1983 14,666 high-risk individuals in the Canton of Zurich, which has one million inhabitants, received 36,234 hepatitis B vaccine injections. The annual number of acute hepatitis B cases dropped from an estimated 220 to 280 in 1981 to 177 in 1982 and 133 in 1983. This drop of 40-50% resulted mainly from reduced numbers of cases among health care workers, drug addicts and homosexuals. Vaccination was a factor responsible for the reduced incidence of hepatitis B among health care workers and drug addicts, high-risk categories substantial proportions of which had been vaccinated. There was no evidence of a secondary protective effect of vaccination extending to other high-risk groups.