The aim of this study was to make a thorough inventory of the histologic features of epithelium and stroma within and adjacent to breast fibroadenomas in 396 cases. Breast fibroadenomas seemed to display a wide spectrum of proliferative and nonproliferative histologic changes. Hyperplasia (excluding mild hyperplasia) within the fibroadenoma was found in 32.3% of cases. Carcinoma in situ (CIS; 5 ductal, 3 lobular) was found in 8 fibroadenomas (2.0%) removed from 6 patients, the youngest of whom was 40 years of age. In 3 cases CIS was not confined to the fibroadenoma, but also involved the adjacent parenchyma. No invasive carcinoma was present within this series. Complex histologic features were seen in 40.4% of cases, mostly in relatively older patients (mean age, 35.4 years). Hyperplasia in adjacent tissue was found in 8.8% of cases, usually in older patients (mean age, 45.5 years). Known risk-elevating lesions in and around breast fibroadenomas occur frequently and mostly in patients older than 35 years. These findings may have consequences for the clinical management of a subgroup of patients with fibroadenoma.