Spindle cell carcinoma of the breast, a variant of metaplastic carcinoma, includes a wide spectrum of lesions with histomorphologic and nuclear features ranging from overtly malignant to mildly atypical. Spindle cell carcinomas with mildly atypical features may resemble fasciitis, fibromatosis, or myofibroblastic tumors and therefore are often misinterpreted as such. A recent study has suggested that spindle cell carcinomas with a dominant fibromatosis-like phenotype, unlike spindle cell carcinomas in general, have no propensity for distant metastasis and should be termed "tumors" rather than "carcinomas." To investigate the question of fibromatosis-like spindle cell breast carcinoma (FLSpCCs) metastatic potential, we studied cases of FLSpCC seen at the University of Texas M.D. Anderson Cancer Center between 1987 and 2000. Clinical, pathologic, and immunophenotypic features were reviewed, with emphasis on biologic behavior and predictors of clinical outcome. Our series included 24 women who ranged in age from 55 to 85 years (mean 66 years). Tumor size ranged from 1.0 to 5 cm (mean 2.8 cm). Most tumors were grossly well defined but had microscopic infiltrative borders. Tumors showed a dominant fibromatosis-like or myofibroblastic-like growth pattern with prominent collagenization. Inflammatory infiltrate was noted in the majority of tumors. Cytokeratin-positive cells were seen in all cases and usually appeared as cords or sheets of polygonal cells; isolated cytokeratin-positive cells were rare. In most tumors immunoreactivity for smooth muscle actin (SMA) was confined to the cytokeratin-negative cells. In five cases intense co-expression of cytokeratin and SMA was noted. None of the tumors showed immunoreactivity for smooth muscle heavy chain myosin, estrogen receptors, progesterone receptors, or HER-2/neu. Ki-67 expression was noted in fewer than 5% of tumor cells. Treatment consisted of local excision (seven cases) or modified radical mastectomy (13 cases). Treatment was unknown in four cases. In patients who underwent axillary nodal dissection, no lymph node metastases were found. Two of the six patients who underwent local excision developed local recurrence. Two patients who underwent modified radical mastectomy developed lung metastases within 2 years after the initial diagnosis. The metastatic tumors were histologically similar to the primary tumors. Our findings indicate that FLSpCCs have the potential for local recurrence and distant metastasis and should be treated accordingly. Because FLSpCCs may be underdiagnosed as benign, the use of immunohistochemical studies, especially for cytokeratins and SMA, is essential in the evaluation of any spindle cell proliferations of the breast.