Fibromatosis (desmoid tumor) can sometimes develop at sites of previous surgery or previous trauma, but it is very rarely detected at sites of prior irradiation. We report two cases of fibromatosis following radiotherapy for Hodgkin's disease. The fibromatoses, developing in the radiation fields, were detected at the posterior area of the neck 11 years after irradiation in one case and in the peritoneal cavity 3 years after laparotomy and irradiation in the second case. The tumors did not differ clinically or histologically from those seen in patients who did not undergo irradiation. Although rare, fibromatosis should enter the differential diagnosis of masses developing in irradiated fields in patients with treated Hodgkin's disease.