The authors reevaluated 10,542 consecutive breast biopsies of women who presented at three Nashville hospitals. Median follow-up was 17 years for 3,398 women (84.4% of patients originally selected for follow-up). Breast cancer relative risks associated with no proliferative disease, proliferative disease without atypia, and atypical hyperplasia were 0.80, 1.4, and 4.0 times that for women from the Cancer in Connecticut data base, respectively (adjusted for age at biopsy, year of biopsy, and length of follow-up). Nulliparous women were at increased risk of breast cancer (relative risk = 1.6; 95% confidence interval (CI) = 1.1-2.2). Women who gave birth to their first child before age 21 years had a relative risk of 0.80, with higher cancer risks associated with later age at first birth. The effect of age at first birth on cancer risk followed a similar pattern within the no proliferative disease, proliferative disease without atypia, and atypical hyperplasia groups. Nulliparous women with atypical hyperplasia had a relative risk of 4.9 (95% CI = 2.7-8.9), while women with no proliferative disease who gave birth before age 21 years had a relative risk of 0.50 (95% CI = 0.19-1.3). Nulliparous women with a family history of breast cancer had a relative risk of 2.7 (95% CI = 1.4-5.2). Women with a family history who first gave birth by age 20, between ages 21 and 29, and after age 30 years had relative risks of 0.53, 2.1, and 4.0, respectively (95% CI = 0.08-3.8, 1.1-3.9, and 1.8-9.6, respectively). Breast size had no effect on cancer risk in women without proliferative disease. However, in women with proliferative disease, small, medium, and large breasts were associated with relative risks of 1.2, 1.4, and 2.1, respectively.