BACKGROUND Apocrine metaplasia is occasionally superimposed on sclerosing adenosis (apocrine adenosis) in breast biopsies, and cytologic atypia is sometimes present (atypical apocrine adenosis). The long term risk of patients developing breast carcinoma subsequent to the diagnosis of this lesion is unknown. METHODS Atypical apocrine adenosis was defined as apocrine adenosis with enlarged nucleoli and a greater than threefold variation in nuclear area. Lesions with recognizable cytoarchitectural patterns of intraductal carcinoma were excluded. Surveillance, Epidemiology and End Results (SEER) data were used as the reference population for calculations of relative risk. RESULTS Thirty-seven women with atypical apocrine adenosis had a mean follow-up of 8.7 years. Four patients developed invasive ductal carcinoma of the breast (3 ipsilateral, 1 contralateral) after a mean of 5.6 years. The relative risk of developing carcinoma was 5.5 (95% confidence interval [CI], 1.9-16). All patients who developed carcinoma were older than age 60 at the time of breast biopsy showing atypical apocrine adenosis, and carcinoma developed at a mean age of 70 years. In the older than 60 years age group (11 patients), the relative risk of developing carcinoma was 14 (95% CI, 4.1-48). CONCLUSIONS Atypical apocrine adenosis confers an increased risk of developing breast carcinoma in women older than age 60, and the risk in younger women is probably low. Some cases of atypical apocrine adenosis may represent in situ apocrine carcinomas that are difficult to diagnose because of the absence of the usual architectural features of intraductal carcinoma.