Twenty-six adult patients with the pathologic diagnosis of desmoid tumor were treated between 1964 and 1983 at the Institut Curie in Paris with megavoltage irradiation. Twenty of these patients (76 percent) had extraabdominal tumors. Definitive surgical resection was performed on nine patients (one received preoperative radiotherapy). At last follow-up 1 1/2 to 10 years after treatment, all of the patients had no evidence of disease. Seven of the nine had follow-up examinations from 5 to 10 years after treatment. Seven patients had postoperative radiotherapy with doses from 4,700 to 6,500 rads (47 to 65 Gy) for either microscopic (three patients) or gross (four patients) residual disease. All but one patient had no evidence of disease from 2 to 8 years after treatment. Nine patients had radiotherapy for recurrent inoperable tumors and six had no evidence of disease from 3 to 20 years after treatment. Recurrences developed in three patients; outside the treatment portal in one, and the other two had received less than 5,000 rads (50 Gy). Clinical regression of tumors after treatment was slow, with complete regression taking up to 2 years. Postoperative radiotherapy with doses of at least 5,000 to 6,000 rads (50 to 60 Gy) was effective in achieving local control of inoperable or incompletely resected tumors, thus the need for repeated resections was avoided. Computerized tomography has greatly improved the assessment of tumor extension and should be used routinely before either operation or radiotherapy to obtain adequate margins and minimize the chance of missing disease.