The authors compare the progression-free survival of patients with unresected or partially resected desmoid tumors treated with radiotherapy with those not given radiotherapy. A retrospective analysis and pathologic review was performed on 14 patients treated at the University of California Los Angeles School of Medicine from 1965 through 1992. Median follow-up was 6 years. The 6-year progression-free survival for irradiated patients was 100%, compared with 50% for those not irradiated (p = 0.04). Of the seven patients irradiated, only two had a complete response and one had a partial response. There was no difference in disease-specific survival between patients irradiated and those not irradiated, because only 1 of 14 patients died of desmoid tumor progression, which caused airway obstruction. This data suggest that radiotherapy may improve the progression-free survival of patients with unresected or partially resected desmoid tumor; however, the number of patients in this series is small. Most patients did not have a complete response to radiotherapy. For patients with tumor adjacent to the airway in the neck or upper thorax, the authors recommend radiotherapy because of the potential for mortality. Otherwise, because tumor progression rarely causes death, one must consider whether the morbidity of treatment would outweigh the morbidity of disease progression.