Although the vast majority of eccrine spiradenomas behave in a benign fashion, 23 cases of malignant transformation have been reported to date. We describe a unique example of malignant eccrine spiradenoma that arose in the right breast of a 68-year-old woman. The quiescent mass, which was present for approximately 50 years, experienced sudden enlargement with erythematous changes of the overlying skin and nipple discharge. Microscopically, the tumor showed the typical features of an eccrine spiradenoma with areas of adenocarcinoma, squamous cell carcinoma, and sarcoma. The sarcomatous component consisted of rhabdomyosarcoma and osteosarcoma. The immunoperoxidase staining revealed p53 protein expression only in the carcinomatous and sarcomatous components. This suggests that accumulation of p53 protein may be an important event in the malignant transformation of spiradenomas. Because of its location and biphasic nature, this malignant eccrine spiradenoma should be distinguished from metaplastic breast carcinoma. To our knowledge, this represents the first carcinosarcomatous transformation of eccrine spiradenoma in the breast. This case led us to conclude that breast tissue, which often undergoes apocrine metaplasia and gives rise to apocrine neoplasms, is also capable of originating benign and malignant tumors with eccrine sweat duct phenotype.