The clinical and pathologic findings of 16 examples of a distinctive stromal tumor of the breast designated as "myofibroblastoma" are reported. Eleven of the 16 patients were men, and the average age at presentation was 63 years. Fourteen were treated by local excision and two by simple mastectomy. None of the lesions recurred or metastasized. The tumors were grossly nodular and well-demarcated from the surrounding mammary tissue. Ducts and lobules were not engulfed by the neoplasm. Microscopically, the lesions were formed by uniform, slender, bipolar spindle cells haphazardly arranged in fascicular clusters separated by broad bands of hyalinized collagen. Ultrastructural examination of four lesions identified a predominance of myofibroblasts. Immunoreactivity for S-100 protein and cytokeratin was absent in the 10 tumors examined, but desmin immunoreactivity was focally present in three lesions. The differential diagnosis of myofibroblastoma includes reactive processes and benign neoplasms such as nodular and proliferative fascititis, fibromatosis, spindle-cell lipoma, neurofibroma, neurilemmoma, and leiomyoma. Malignant neoplasms such as stromal sarcoma, malignant fibrous histiocytoma, and spindle-cell or metaplastic carcinoma should not be confused with a myofibroblastoma. The clinical significance of this entity lies primarily in its recognition as a distinctive benign neoplasm.