Over one million American women have a benign breast biopsy annually. Sclerosing adenosis (SA) is a common, but poorly understood benign breast lesion demonstrating increased numbers of distorted lobules accompanied by stromal fibrosis. Few studies of its association with breast cancer have been conducted, with contradictory results. We studied SA in the Mayo Benign Breast Disease (BBD) Cohort, which includes women who had benign biopsies at Mayo-Rochester 1967-2001. Breast cancer risk in defined subsets was assessed using standardized incidence ratios (SIRs), relative to the Iowa Surveillance, Epidemiology, and End Results registry. This BBD cohort of 13,434 women was followed for a median of 15.7 years. SA was present in 3,733 women (27.8 %) who demonstrated an SIR for breast cancer of 2.10 (95 % CI 1.91-2.30) versus an SIR of 1.52 (95 % CI 1.42-1.63) for the 9,701 women without SA. SA was present in 62.4 % of biopsies with proliferative disease without atypia and 55.1 % of biopsies with atypical hyperplasia. The presence of SA stratified risk in subsets of women defined by age, involution status, and family history. However, SA does not further stratify risk in women diagnosed with other forms of proliferative breast disease, either with or without atypia. SA is a common proliferative lesion of the breast which, as a single feature, conveys an approximate doubling of breast cancer risk. Its role in breast carcinogenesis remains undefined; its presence may aid in risk prediction for women after a breast biopsy.