The tetanus and diphtheria vaccination programme in Finland has been running for 50 years. After primary doses, tetanus boosters have been offered to men in military service and decennial boosters recommended for all through the adult life. For 30 years a diphtheria booster was only offered to men in the military service. Not until 1989 diphtheria-tetanus (dT) and diphtheria (d) booster vaccines for adolescence and adults were introduced. In this study serum samples of 990 subjects from 30 years of age, participating in a population survey in 2000-2001, were used to assess the tetanus and diphtheria antitoxin concentrations. More than 70% of the adults up to 50 years of age were fully protected (antitoxin concentrations >0.1 IU mL) against tetanus and diphtheria. Of these adults more that 76% had antitoxin concentrations >1 IU/mL against tetanus, indicating long-term protection but also an increased risk for hyperimmunisation. A comparison of this study and two immunogenicity studies conducted in Finland in 1987-1988 and 1995-1996 shows the impact of an active decennial dT adult booster programme in a country with a high primary tetanus and diphtheria vaccination coverage in infants since the 1950s. Recommendations for limited decennial boosters by increase the time interval between dT boosters up to 20 years as suggested by this study and also studies performed, e.g., in Denmark and Portugal should be considered. Finnish adults born before 1930 should, however, still be vaccinated with decennial boosters, especially against tetanus.