The clinical and pathologic features of 26 examples of a histopathologically distinct form of metaplastic carcinoma of the breast are reported. All neoplasms had overt carcinoma with direct transition to a cartilaginous and/or osseous stromal matrix without an intervening spindle cell zone or osteoclastic giant cells. Therefore, we designate this distinctive form of metaplastic carcinoma as "matrix-producing carcinoma" (MPC). All patients were women, the average age was 58 years, and all patients were eligible for a minimum of 5 years follow-up (mean follow-up period, 8.6 years). Twenty-three patients were treated by a form of mastectomy and three were treated by local excision. The 5-year survival rate for patients following mastectomy or partial mastectomy was 70%, contrasted with 50% for patients treated by local excision. The cumulative 5-year survival rate for MPC was 68%. All of the nine lesions that recurred did so within 2.5 years of initial therapy. Eight of these patients (89%) died from tumor within 4 years of initial therapy. The ninth was alive at last contract. Radiation and chemotherapy were of limited effectiveness. Significant features of the neoplasm associated with progression were large size, diffuse cellularity of the stromal matrix, and atypical cartilaginous metaplasia. Ultrastructural examination of one case and immunohistochemical evaluation of 12 cases revealed MPC to have myoepithelial characteristics.