A small but growing number of radiation-induced sarcomas after breast-conserving surgery for carcinoma have been reported. Because breast-conserving surgery followed by irradiation is becoming increasingly popular, the potential for the emergence of these sarcomas is growing. Unfortunately, because of postirradiation changes in the affected breast, detection of a new lesion can be difficult, resulting in a delay in diagnosis. This case history describes a 48-year-old woman in whom a malignant fibrous histiocytoma was discovered 5 years after she had had a lumpectomy followed by radiation therapy for infiltrating ductal carcinoma. Although the exact mechanism of radiation-induced carcinogenesis is unclear, one theory is discussed. Also discussed are the histologic composition of the malignant fibrous histiocytoma, its latency period, and the best treatment modality for radiation-induced sarcomas.