Twenty-nine patients (thirty sites) who had histologically confirmed aggressive fibromatosis were treated with radical courses of radiation at the University of Florida between March 1975 and February 1986. The minimum length of follow-up was two years; 76 per cent of the patients were followed for more than five years. Twenty-seven sites received doses of at least 5000 centigrays (one centigray equals one rad). Twelve patients were treated twice a day. Fourteen sites were treated with radiation postoperatively for known or presumed microscopic quantities of residual aggressive fibromatosis; in eleven, the disease was locally controlled. Sixteen sites were treated postoperatively for known grossly apparent residual disease; in fourteen, the disease was locally controlled. Over-all, aggressive fibromatosis was controlled in twenty-five (83 per cent) of the thirty sites. The six-year actuarial rate of local control was 79 per cent. The five local recurrences occurred at four, eleven, thirty-four, sixty-one, and sixty-eight months after the initiation of radiation therapy. Two of the five failures occurred in the high-dose radiation field in patients who were treated for grossly apparent disease. The remaining three failures occurred at the margin of the irradiated field in patients who were treated for assumed microscopic quantities of residual disease. There was no apparent difference in local control between patients who were treated for primary (previously untreated) aggressive fibromatosis and those who were treated after one or more recurrences. Comparison of the results of radiation therapy with published data on operative treatment shows that local control substantially improves with postoperative radiation therapy when operative margins are less than wide.(ABSTRACT TRUNCATED AT 250 WORDS)