True adenomas of the breast are cellular epithelial lesions, which may be confused with carcinoma and are difficult to classify. We have reviewed 28 adenomas of the breast and have developed specific diagnostic criteria, as well as a classification of breast adenomas based on histologic features. Electron microscopic studies confirm that adenomas have ultrastructural features similar to normal breast epithelium and are not variants of fibroadenomas. The association between lactating adenomas and pregnancy has been confirmed; however, clinicopathologic correlation does not suggest a relationship between adenomas and oral hormonal medication. Although adenomas are benign, this study includes the third reported case of an infiltrating carcinoma associated with a lactating adenoma.