The clinical and pathologic features of 70 examples of carcinosarcoma (CS) of the breast are reported. Thirty-three neoplasms had infiltrating carcinoma, seven had in situ carcinoma, and 28 had both admixed or contiguous with the sarcomatous component. Squamous carcinoma, present in 15 neoplasms, was the exclusive epithelial component of two. The admixed carcinoma often appeared distinct from the sarcoma component; however, at high magnification transitional differentiation zones and more subtle merging of infiltrating carcinoma with sarcoma were present in most neoplasms. A total of 40 neoplasms were studied by immunohistochemistry for keratins, EMA, vimentin, S-100 protein, and actin. The sarcomatous component in 55% of CS was immunoreactive for keratin, and 98% were immunoreactive for vimentin. A majority were also immunoreactive for actin (77%), and S-100 protein (55%). Ultrastructural examination of the sarcoma in eight neoplasms yielded variable nonspecific findings compatible with sarcoma. These findings indicate biphasic differentiation by cells possessing epithelial and mesenchymal characteristics and suggest myoepithelial origin or differentiation. The cumulative 5-year survival rate for CS was 49%, worse than for other forms of metaplastic carcinoma. The respective 5-year survivals for TNM clinical Stages I, II, and III were 100%, 63%, and 35%. Of patients with axillary dissection, 26% had metastases to axillary lymph nodes with carcinoma as the most frequent component to metastasize. Metastasis was an ominous sign as 33 of 34 patients who developed metastases died from tumor. Local recurrence was not as ominous as 40% who had only local recurrence subsequently died from tumor. Size and microscopic circumscription were also significant prognostic factors.