A group of 1,489 white women were treated in a private surgery practice from 1940 through 1975 for biopsy-proved benign breast disease, and 1,441 were followed through 1976 for the development of breast cancer. Average duration of follow-up was 12.9 years for a total of 18,617 person-years of observation. Information was collected from a set of questions devised in 1941 and asked of all subjects at the time of their initial office visit, follow-up interview conducted in 1976, and a standardized histology review of the slides from the initial benign lesions and the subsequent cancers. The current pathology review indicated that 66 of the women developed breast cancer. The incidence rate was 3.55 per 1,000 person-years, which is 2.10 times that of the general population. When multiple disease types and other variables were controlled for, excess risk of breast cancer was related to the presence of fibrocystic disease. In women with fibrocystic disease, excess risk was particularly related to the presence of epithelial hyperplasia and/or papias not related to the presence of fibroadenoma alone, but it was related to the presence of fibroadenoma in women with concomitant fibrocystic disease. The excess risk was also directly related to the estimated size of the initial benign mass and was greater for women with bilateral than with unilateral benign lesions.