The authors studied the relation between benign breast disease and subsequent breast cancer in 16,692 women with biopsy-diagnosed benign breast disease who had participated in the Breast Cancer Detection Demonstration Project throughout the United States. Women were classified into one of five benign breast disease categories: atypical hyperplasia, proliferative disease without atypia, nonproliferative disease, fibroadenoma, and other benign breast disease. A total of 485 incident cases of breast cancer were identified in the women from August 1973 to February 1986 after a median follow-up period of 8.3 years from the diagnosis of benign breast disease. Age-adjusted incidence rates were calculated for benign breast disease types stratified by family history and calcification status. Relative risk (RR) estimates of breast cancer for women in the five benign breast disease categories, compared with the screened women who did not develop recognizable breast disease (normal subjects), were computed using the proportional hazards model. Results indicated that risk was associated with the degree of epithelial atypia. Over all age groups, women with nonproliferative disease, proliferative disease without atypia, and atypical hyperplasia displayed progressively increasing risks of 1.5, 1.9, and 3.0, respectively, compared with normal subjects, with 95% confidence intervals (CI) exceeding unity. Particularly high risk was seen among women under age 46 years with atypical hyperplasia (RR = 5.7, 95% CI 3.0-10.6). Women with fibroadenoma as the only indication of their benign breast disease had a relative risk of 1.7, with a lower 95% confidence limit of 1.0. No increased risk was seen for women with other benign breast disease. Positive family history (RR = 1.8) and calcification (RR = 1.2) significantly increased a woman's risk proportionately over the risk associated with each benign breast disease subtype. The authors conclude that the risk of developing breast cancer varies by category of benign breast disease and is directly related to the degree of epithelial atypia.