Metaplastic carcinomas of the breast are a heterogeneous group of neoplasms ranging from tumors with a predominant component of overt carcinoma and focal mesenchymal differentiation to keratin-positive tumors with pure sarcomatoid morphology. We examined the clinicopathologic features of 22 patients previously diagnosed at M.D. Anderson Cancer Center with metaplastic carcinoma of the breast with pure or almost pure sarcomatoid morphology. Patients were included in the study if their tumors had sarcomatoid morphology and: 1) an invasive carcinomatous component identifiable on hematoxylin and eosin stains comprising less than 5% of the invasive tumor; or 2) associated ductal carcinoma in situ; or 3) immunohistochemical expression of keratin in the sarcomatoid areas. Patients with low-grade fibromatosis-like metaplastic tumors and those who received neoadjuvant chemotherapy were excluded. Axillary lymph node dissection or limited axillary node excision was performed in 17 patients, including 1 patient who had a sentinel lymph node biopsy. Lymph node involvement occurred in only 1 patient and consisted of a single 3.5-mm metastasis. Clinical follow-up was available for 21 patients and ranged from 4 months to 155 months (median follow-up, 35 months). Ten patients experienced local relapse, including 7 of 11 patients treated with breast-conserving surgery, and 9 developed distant metastases, most frequently to the lungs. These findings suggest that metaplastic sarcomatoid carcinomas that lack or have only a minimal overt invasive carcinomatous component have a biologic behavior similar to that of sarcomas. In addition to systemic treatment, early aggressive local therapy is recommended, as these patients have a high rate of local relapse.