Twenty-six cases of breast carcinoma demonstrating pseudosarcomatous metaplasia are described, and their clinical and pathologic aspects are correlated. The metaplastic elements include mature and immature bone, cartilage, myxoid stroma, loose fibromyxoid and dense spindle-cell stroma, and anaplastic stroma with giant cell features. Light and electron microscopic examination demonstrated an orderly sequence of transformation and dedifferentiation of epithelial cells to become undifferentiated mesenchymal cells. These tumors are more aggressive than purely epithelial carcinomas. The overall survival rate was 44%. Five-year survival figures for TNM Stages I, II, and III lesions were 56%, 26%, and 18%, respectively. The incidence of lymph node metastasis was 25% despite the large size of many of these tumors. Systemic metastases replicated the range of metaplastic elements seen in the primary site. Patients with tumors composed predominantly of pseudosarcomatous elements had worse prognoses than those with predominantly epithelial components (28% versus 62%, 5-year survival).