Atypical medullary carcinomas and carcinosarcoma have unique histopathological features. Here we present a case with a breast malignancy that had pathological characteristics of both. A 54-year old patient with a malignant breast mass received 6 cycles of adriamycin-based chemotherapy, followed by 3 cycles of paclitaxel monotherapy, and had a poor clinical response to treatment. A modified radical mastectomy was performed. The pathological diagnosis was complicated by an inability to distinguish between atypical medullary carcinoma and carcinosarcoma. The findings included a tumor that was well-circumscribed, high grade and a syncytial growth pattern as well as biphasic sarcomatous and carcinomatous characteristics. In conclusion, atypical medullary carcinoma and carcinosarcoma of the breast have entirely different prognoses and should be managed differently. Both should be treated by surgical resection, and additional therapy should be considered based on the cancer with the poorer prognosis.