The clinical and pathologic findings in 29 patients whose primary breast neoplasm manifested the microscopic pattern of spindle cell carcinoma or extensive squamous or pseudosarcomatous metaplasia were studied. In several of the tumors, the diagnosis of primary sarcoma or squamous cell carcinoma was excluded only after a prolonged search for evidence of invasive ductal carcinoma. The paucity of axillary lymph node metastases and the circumscription of these neoplasms belied their aggressive clinical behavior. The size of the neoplasm at the time of initial treatment best correlated with prognosis, since the majority of patients whose carcinoma was less than 4 cm in diameter pursued a favorable course. The lack of correlation of the microscopic pattern of these neoplasms with prognosis, as well as the presence of apparent overlapping microscopic findings, supports the concept that they are variants of a single entity.