The clinical and pathologic features of 37 examples of intraductal apocrine carcinoma are reported along with criteria for distinguishing apocrine intraductal carcinoma (AIDCA) from apocrine hyperplasia. A majority of AIDCAs (32 cases) displayed both cytological atypia and intraluminal necrosis. Five AIDCAs lacked intraluminal necrosis but had at least subtle atypia with either epithelial bridging or a solid, mosaic-like growth pattern involving one or more ducts exceeding 2 mm in maximum cross-sectional diameter. Twenty-nine patients had mastectomy. Eight women had excisional biopsy or quadrantectomy only; two of these developed recurrences 2 and 4.5 yr following their biopsies. The original AIDCA in both patients was of the comedo type and neither woman had received any further treatment following biopsy. Both women were alive 4 and 6 yr after their respective mastectomies. The follow-up for the entire group ranged from 2.5 to 20 yr with a mean of 11.4 yr. One of the eight women died from breast cancer 4 yr after the diagnosis of her AIDCA; she had had mastectomy for a node-negative infiltrating nonapocrine carcinoma in the contralateral breast 4 yr before her AIDCA, however, and probably died from progression of the invasive carcinoma of the contralateral breast. In the absence of necrosis and significant atypia, a diagnosis of AIDCA should be made cautiously.