Three strategies regarding hepatitis B virus vaccination were compared by decision analysis: no vaccination, immediate vaccination, and vaccination after two years. The potential advantage of waiting two years is to learn whether serious side effects of the vaccine will become evident. For example, it was found that immediate hepatitis B vaccination of 100,000 surgical house officers with a 5 percent annual attack rate for five years would, compared with no vaccination, prevent 4,092 cases of icteric hepatitis, 335 cases of chronic active hepatitis, and 15 deaths from fulminant hepatitis. For a strategy of waiting two years, the number of cases prevented would decrease by about 40 percent. Persons in groups with an annual attack rate lower than 5 percent appear to benefit from vaccination. The known health risks of hepatitis B virus vaccination are low, and the hypothesized risks would have to be frequent to justify delay in vaccination. From an individual perspective, even persons at low risk of hepatitis B virus infection should seriously consider immediate vaccination.