To solve the problem of diagnosing apocrine carcinoma (APCA) through distinguishing it from benign apocrine metaplasia with atypia (APMA). The study group consisted of five histologically confirmed cases of uncommon infiltrating apocrine carcinoma and a case of noninfiltrating apocrine carcinoma of the breast by aspiration biopsy cytology. The control group consisted of 103 cases of benign apocrine metaplasia with no atypia (APMN), 4 cases of APMA and 34 cases of common-type adenocarcinoma that were encountered in 662 breast aspirations from 1988 to 1992 at Hekinan Municipal Hospital. In APCA the average age of patients (65 +/- 17.7 SD)(mean +/- SD) was more than 20 years older than APMA, and APCA generally measured > or = 2 cm or more in diameter as compared to < or = 2 cm in APMA lesions. In APCA the apocrine cells had high cellularity, with the cells occur singly and with syncytia in the background. Numerous degenerated apocrine cells and characteristic cell detritus were found. APCA was also characterized by apocrine cells with more marked nuclear abnormalities, including hyperchromasia and irregular nuclear shape, frequently with irregular nucleoli; more nuclei measure > or = 12 micron in diameter than in APMA. These findings, however, were present only to a mild degree in APMA, if at all. APMA may coexist with APCA. If APMA is diagnosed, an open biopsy should be performed to distinguish it from APCA.