The issue of risk of breast cancer subsequent to benign breast diseases (BBD) remains controversial. To determine whether this risk varies by histologic type of BBD, degree of atypia (as measured by the Black-Chabon method), age at BBD biopsy, or length of follow-up, the authors followed 2,731 women in the San Francisco Bay Area for an average of 16 years after their first occurrence of biopsy-proven BBD between 1948 and 1973. The cohort's age-adjusted rate of breast cancer was 1.8 times that of the general population (95% confidence interval (Cl) 1.6–2.2). Among the different histologic types of BBD, rates were greatest for intraductal papiltoma (rate ratio (RR) = 3.9, 95% Cl 2.0–7.4) and adenosis (RR = 2.5, 95% Cl 1.2–4.9). Rates were also significantly elevated for fibrocystic BBD (RR = 1.5, 95% Cl 1.1–1.9). Risk increased directly with degree of atypia, rising from a rate ratio of 1.8 among women with Black-Chabon scores of 1 and 2 (95% Cl 1.5–2.2) to 7.2 among women with a Black-Chabon score of 5 (95% Cl 1.8–28.9). Among women with the higher-risk types of BBD, rate ratios increased with age at BBD biopsy and decreased with increasing length of cancer-free follow-up. These findings are interpreted in the light of possible biases affecting this and other studies and recent advances in knowledge regarding neoplastic lesions and breast cell kinetics. Am J Epidemiol 1992;135:619–31.