In 196 randomly selected persons with documented complete primary vaccination against tetanus 12-30 years earlier, antitoxin concentration in serum relative to time since last vaccination was studied. In the group of non-revaccinated persons an exponential decrease in antitoxin concentration with time since primary vaccination was found. 25-30 years after primary vaccination, 28% were unprotected (antitoxin concentration in serum below 0.01 IU/ml). In the group that received a single revaccination earlier, a similar exponential fall in antitoxin concentration was found, but all those revaccinated up to 23 years earlier were still protected. 82 persons who had not been revaccinated earlier were revaccinated. All those revaccinated less than 20 years after primary vaccination attained very high antitoxin concentrations (above 6 IU/ml) corresponding to a duration of immunity of at least 20 years. It was suggested that a revaccination program consisting of a single revaccination in childhood and thereafter every 20 years is sufficient to maintain immunity.