To investigate minimal requirements for tetanus revaccination to secure continuous protection, recently recommended by WHO, 637 subjects with documented vaccination history were studied. Antitoxin concentration in serum relative to time corresponded to a steep decline in the first years after vaccination continuing exponentially. By multiple regression analyses duration of immunity after three-dose primary vaccination was calculated to be 5 years (upper 95% confidence limit of estimated risk of serum antitoxin concentration below 0.01 IU ml-1 still less than 0.1%). Serum antitoxin concentration relative to time after revaccination depended upon age at revaccination and interval from primary vaccination. When given in childhood 5 years after primary vaccination revaccination was calculated to offer protection for approximately equal to 21 years, but protection was considerably shorter when given to the elderly. Cross-sectional and longitudinal analyses were compared and statistical approaches were introduced which may be generally applicable for evaluation of vaccination programmes. It was concluded that a vaccination programme consisting of primary vaccination in infancy and one revaccination 5 years later will secure continuous protection to about the age of 25 years. This is considerably simpler than programmes recommended in many countries, in which risk of hyperimmunization is apparent.