Several instances of fatal postoperative tetanus have been reported, and it has recently been stated that failure to secure adequate immunization for surgical patients may have medicolegal repercussions. In order to investigate according to which criteria for the immunization surgical patients can be considered protected, 178 patients who were more than 50 years of age and who were admitted electively during one year to two surgical departments were studied. To investigate immunity relative vaccination history, all patients admitted during the first two months were offered participation in this study. One hundred and twenty-seven (86 per cent) of these patients were studied by detailed registration of vaccination history and determination of antitoxin concentration in serum. Forty-seven per cent had never been vaccinated against tetanus and 67 per cent had antitoxin concentrations in serum below the protective level. To investigate in detail the duration of immunity and the effect of revaccination, patients with documented complete primary vaccination only were studied during the entire period. An additional 51 patients have been entered into the study during the last ten months. Antibody concentration in the serum corresponded to a continuous fall off in immunity after vaccination. Patients who had received a complete three dose primary vaccination within the last five years or a revaccination within the last ten years were well protected. Forty-four patients received revaccination during the study and were controlled by blood sample four weeks later. Every patient obtained satisfactory antibody concentrations, and it was concluded that no special requirements in recommendations for vaccination are needed, even for elderly patients. Thirteen patients who underwent extensive surgical procedures had serial blood samples taken every three days. Antitoxin concentrations in serum increased sigmoidally with time, and the slow initial increase suggested that revaccination should optimally be given some days before the operation to assure protection in the first few postoperative days.