The prognosis after long-term survival was analyzed for 645 patients with oral squamous cell carcinoma treated primarily with radiotherapy. In contrast to the fact that the main cause of death within five years was related to the primary disease (92%), the estimated causes of death after five years were related to the primary disease in 22% and to other malignant tumors in 15%; this was particularly so in the middle age group (29%). As for the patients who showed local recurrence after long periods the possibility of radiation-induced cancer might be considered; however, it was difficult to distinguish radiation-induced cancer from late recurrence.