In a double-blind trial, we randomly assigned 1330 high-risk health care personnel to receive three 20-micrograms doses of hepatitis B vaccine or placebo. Among vaccine recipients 58% responded within 1 month and 97% within 9 months; there was no difference in immune response to the vaccine between men and women. Efficacy was evaluated after a mean follow-up of only 13.2 months, just before the vaccine was released commercially. Five hepatitis B infections were identified in placebo recipients and one in a vaccine recipient. Although the number of infections was too small to allow confident conclusions about protective efficacy of the vaccine, we saw a 67% reduction in the need for hepatitis B immune globulin after accidental hepatitis B inoculation in the vaccine group (relative risk, 5.08; 95% confidence intervals, 1.3 to 19.9). Minor side effects occurred with equal frequency after vaccine (28.7%) and placebo (27.2%) injections; no participant had a severe adverse reaction. Vaccination with the 20-micrograms hepatitis B vaccine was highly immunogenic and safe in health care workers.