Sclerosing breast lesions with cytologically atypical apocrine metaplasia present a difficult diagnostic problem. The clinical significance of these lesions has not been established although, historically, apocrine metaplasia and sclerosing adenosis have been regarded as totally benign. To further assess these lesions, we studied 51 patients with atypical apocrine metaplasia in sclerosing lesions of the breast, obtaining an average follow-up of 35 mo (12 to 76 mo). For convenience, these have been grouped together as atypical apocrine sclerosing lesions (AASL). The average age at diagnosis was 58 yr. The lesions tended to be small (67% less than 1.0 cm) and usually were detected by mammography (78%). Four patients had an ipsilateral mastectomy following a biopsy of AASL that was interpreted as carcinoma or "precancerous" by the referring pathologist. Two patients had nonapocrine intraductal and infiltrating duct carcinoma in previous contralateral mastectomy specimens. None of the 47 women with an intact breast developed breast carcinoma during the follow-up period. Immediate treatment with surgery and/or radiation is not indicated. The long-term clinical implication of these lesions is yet to be determined; therefore, continued clinical observation of AASL patients is advisable.