In an effort to vaccinate as many high-risk individuals as possible against hepatitis B, 10,484 persons received a total of 27,818 injections of H-B-Vax (Merck Sharp & Dohme) or HEVAC B (Sanofi/Pasteur) between January 1982 and January 1983. They represented 1.1% of the total population of the canton of Zurich including approximately 70% of all medical and dental personnel, a great proportion of eligible high-risk patients and contacts of HBs Ag carriers, but not more than 10% of drug addicts and promiscuous homosexuals. Twelve to 17% of the vaccine injections were accompanied by side effects which were benign in nature and transitory. Six vaccinated individuals developed hepatitis B, all within 60 days after the first vaccine dose. One hundred and seventy-seven cases of hepatitis B were recorded in 1982, 109 in the first half of the year and 68 in the second half. It was concluded that the vaccination campaign provided protection for medical and dental personnel. There was still no conclusive evidence of a vaccine-related drop in the incidence of hepatitis B in other high-risk groups vaccinated or - as a secondary effect - in other non-vaccinated populations.