The clinical and pathologic features of 29 examples of mammary metaplastic carcinoma with osteoclastic giant cells (OGC) in the stroma are reported. A bland spindle cell or sarcomatous component dominated these neoplasms, although infiltrating duct carcinoma was present in 23 cases, and intraductal carcinoma was present in six cases. In all 29 neoplasms, the carcinoma was admixed or contiguous with the stroma. Osteoclastic giant cells were admixed within the cellular stroma, and were intimately associated with prominent thin-walled vessels. Hemorrhage and hemosiderin deposition were common. Osteoclastic giant cells were immunoreactive for vimentin and, to a lesser extent, actin, and uniformly not immunoreactive for keratins, confirming their mesenchymal nature. The stromal component of 63% of neoplasms tested was immunoreactive for keratin, 33% was immunoreactive for epithelial membrane antigen, 54% reacted for S-100 protein, 84% reacted for actin, and 100% was immunoreactive for vimentin. Nineteen neoplasms had osteoid, bone, or cartilage, but these were a prominent component in only five neoplasms and OGC were not limited to these areas. The disease-specific cumulative 5-year survival rate for patients with metaplastic carcinoma with OGC was 68%, similar to rates for patients with matrix-producing carcinoma (68%), spindle cell carcinoma (64%), and squamous carcinoma of ductal origin (63%), but notably different from that of patients with carcinosarcoma (49%). Of 17 women with axillary node dissection, only two had metastases. Eleven women developed distant metastases, most commonly to the lungs. Metastasis present at or following initial surgery was an ominous sign, as all 11 women with metastases died from tumor. Size and microscopic circumscription were significant factors in predicting disease progression.