The purpose of this study was to determine retrospectively any changes in the dentition, saliva secretion, mucous membranes and the lower jaw resulting from the radiation treatment given to patients with cancer of the tongue or floor of the mouth. The study covers 29 patients treated at the Department of Radiotherapy and Oncology of Helsinki University Central Hospital between 1972 and 1981. 23 of the patients were treated for cancer of the tongue and the remaining 6 for cancer of the floor of the mouth. The radiation doses ranged from 5300 to 6700 rad. Clinical examination revealed that the number of carious teeth doubled as a result of radiotherapy, while the incidence of cervical lesions increased by a factor of 10, both compared with the controls. About 70% of the patients studied were found to have either moderate or extensive fungal growth. Stimulated saliva secretion had fallen to about 20% and resting saliva secretion to about 30% of the values for the control group. Radiography revealed pathological changes in the mandible in 34% of the patients. Clinically, 2 patients were found to have extra-oral fistulae. In 1 of these, the spontaneously fractured end of the mandible was visible for several cms. 3 of the patients had undergone hemimandibulectomy because of osteoradionecrosis.