The necessity of aggressive therapy for desmoid tumors has not been clearly established. To evaluate the therapeutic value of adequate resection and radiation therapy, we conducted a retrospective study of 138 patients treated from 1965 through 1984. Univariate analysis revealed five factors predictive of local failure: (1) age between 18 and 30 years, (2) presentation with recurrent disease, (3) partial or limited margin excision, (4) tumor at or close to the microscopic margin of resection, and (5) radiation therapy not administered for gross residual disease. Multivariate analysis identified two of these factors as having independent predictive value for recurrence: (1) presentation with recurrent disease and (2) less-than-adequate margins of resection. The five-year survival probability was 92%, but 11 of the 138 patients died as a consequence of locally uncontrolled tumor. These findings confirm that desmoid tumors are malignant soft-tissue neoplasms that warrant aggressive therapy.