The clinicopathologic features of 32 metaplastic carcinomas with heterologous osteocartilaginous elements are reported. Each neoplasm consisted of invasive adenocarcinoma accompanied by a cartilaginous or osseous component. In 10 neoplasms, this consisted of cartilage and in 2 the heterologous element was osteoid or bone exclusively. The remaining 20 neoplasms contained a mixture of cartilaginous and osseous components. All patients were women; mean age was 56 years. Twenty-four patients were treated using mastectomy and eight by local excision. Twenty-six patients underwent axillary lymph node dissection. Lymph node metastases were detected in 6 of the 26 (23%) patients who underwent axillary dissection. Clinical follow-up was available for 29 of 32 patients (91%). Local recurrence or distant metastases developed in 6 patients (21%) within 2 years of initial treatment, and 4 of these patients died of metastatic carcinoma. The overall 5-year survival rate was 60%. When compared with control patients with infiltrating duct carcinoma, the group with metaplastic carcinoma tended to have a more favorable prognosis after adjustment for nodal status and tumor size. The prognosis of patients with metaplastic mammary carcinoma with heterologous osteocartilaginous elements is dependent on tumor stage at diagnosis. Immunohistochemical studies for 34BE12, p53, retinoblastoma protein, HER/2neu (polyclonal), epidermal growth factor receptor, and cyclin D1 were performed in 18 cases. Positive immunohistochemical staining was found as follows: 34BE12: n = 13 (72%); p53: n = 11 (61%); retinoblastoma protein: n = 12 (66%); HER2/neu: n = 2 (11%); epidermal growth factor receptor: n = 7 (38%); and cyclin Dm: n = 5 (28%). Positive staining for 34BE12 was observed in the carcinomatous component in 5 (38%) of the neoplasms, in the metaplastic component in 2 (15%), and in both elements in 6 (64%). A p53 staining was observed in the carcinomatous component exclusively in 4 (36%) of 11 p53-positive tumors. No disparity in p53 staining was noted between the epithelial and metaplastic elements in the other p53-positive tumors. Expression of these markers did not correlate with clinicopathologic features such as patient age, tumor size, tumor type, relative proportion of metaplastic elements, and axillary nodal status and was not predictive of disease-free survival.