In order to successfully treat tetanus one has to be aware of its causes. Therefore, sources of tetanus were examined in a series of 106 patients with tetanus diagnosed in Finland during the period 1969-1985. The primary injuries which were identified in 100 (94.3%) of them included puncture wound, minor injury, excoriation, animal bite, cut wound, open fracture, burn, undefined wound and other sources. The latter group included nine patients in two of whom tetanus was caused by an operation on a closed injury and in one patient by a decubitus ulcer during hospital care. Six patients had an unusual source of tetanus: frostbite, poor teeth, ulceration in the gum under a denture, infected molluscum contagiosum, a scratched atheroma cutis and an infected granuloma pyogenicum. Two of these nine patients died; the mortality in the whole series was 12 patients (11.3%). It is important to be aware of even the rare sources of tetanus, as the rarity or absence of a primary injury may delay the diagnosis and treatment of tetanus and worsen the prognosis of the patients.