Abstract The potential role of p16 INK4a methylation in breast cancer is controversial whereas there are no data on fibroadenoma. To assess if inactivation of p16 INK4a by promoter hypermethylation occurs in this hyperproliferative benign breast lesion or, on the contrary, it is strictly related to the carcinogenic process, we have tested the different histological components of 15 cases of fibroadenoma and the intraductal and infiltrating components of 15 cases of carcinoma and their adjacent non‐tumoral epithelium. All samples were obtained by laser‐assisted microdissection. The relationship between promoter methylation status, immunohistochemical protein expression and ki67 proliferative activity was evaluated for each lesion. Our data demonstrate that hypermethylation of p16 INK4a promoter is a common event occurring at similar frequency in all the different histological areas of the benign and malignant breast lesions taken into exam. Conversely, protein p16 expression, although heterogeneously distributed within the section, is considerably higher in breast carcinoma as compared to fibroadenoma in both tumoral and non‐tumoral epithelia and stroma. The protein localization was almost exclusively nuclear in fibroadenoma and non‐tumoral epithelia whereas, in carcinoma, the staining was both nuclear and cytoplasmic or cytoplasmic alone. Furthermore, in a subset of fibroadenoma with higher proliferative activity, p16 protein expression was substantially decreased as compared to those showing lower proliferation. We did not observe this association in carcinomas. Our data demonstrate that the hypermethylation of the p16 INK4a promoter is not specifically associated with malignancy and that, on the contrary, the overexpression of p16 and its cytoplasmic sequestration is a feature of breast carcinoma. © 2004 Wiley‐Liss, Inc.