Breast apocrine carcinoma is a rare malignancy characterized histologically by a predominance of acidophilic tumor cells exhibiting apocrine metaplasia. Apocrine tumors represent only about 0.4% of all breast cancers. Many cases are diagnosed at stage I or II; thus, the prognosis is better than that for other types of breast cancer. Here we present a 57-year-old female patient with primary apocrine breast carcinoma that was incidentally discovered by endometrial cytology. The patient had undergone routine uterine cancer screening at the age of 56 with resulting positive endometrial cytology. Subsequent histological examination of an endometrial biopsy revealed suspected metastatic uterine adenocarcinoma. Further evaluation revealed a 3-cm mass in the left breast, which was histopathologically diagnosed as breast cancer. Dual procedures were performed at the departments of gynecology and breast surgery. The patient underwent a modified radical mastectomy of the left breast and left axillary lymph node dissection. Laparotomy revealed multiple adhesions throughout the peritoneal cavity that suggested invasive metastatic cancer, and a right adnexectomy was performed. Histopathological examination of the resected left breast and right ovary tissues revealed apocrine carcinoma. Thus, stage IV breast cancer was diagnosed. The patient finished 6 cycles of paclitaxel and is now under observation on an outpatient basis. In this patient, breast cancer was discovered incidentally through a detailed work-up after a positive result of endometrial cytology. Uterine cancer screening has the potential, although rare, to detect metastatic cancer originating elsewhere. This should be taken into consideration during the work-up due to positive endometrial cytology.