Specific histologic types of benign breast disease (BBD) may increase breast cancer risk. The authors analyzed data from a population-based, case-control study of women aged 20 to 54 with newly diagnosed breast cancer and control subjects randomly selected from the general population. A panel of pathologists classified the histologic findings of biopsy slides for 433 women with breast cancer and 261 control subjects, all of whom had a history of biopsy for BBD, as to the presence of epithelial hyperplasia, atypia, and other histologic features. When compared with women who had never had a breast biopsy, women with BBD without hyperplasia had an odds ratio of 1.5 (95% confidence limits [CL] 1.3 to 1.9), women with hyperplasia without atypia had an odds ratio of 1.8 (CL = 1.3, 2.4), and women with hyperplasia and atypia had an odds ratio of 2.6 (CL = 1.6, 4.1). Fibroadenoma was an independent risk factor for breast cancer (odds ratio = 1.7; CL = 1.1, 2.5). These findings suggest that women with BBD with epithelial hyperplasia either with or without atypia and women with fibroadenoma should be monitored carefully because of their elevated risk for breast cancer.