Background. Women with proliferative breast disease (PD) have been observed to have an increased risk of breast cancer. The authors evaluated the effect of PD on breast cancer risk in a case–control study among participants of the Breast Cancer Detection Demonstration Project (BCDDP). Methods. More than 280,000 women were screened in the BCDDP at 29 centers. Study subjects were selected from BCDDP participants who underwent biopsy that revealed benign breast tissue. There were five BCDDP centers for which histologic slides were available on more than 85% of the benign biopsy specimens. Case patients for this study were the 95 women from these five centers who had breast cancer develop during follow‐up. Two matched control patients who did not have breast cancer develop were selected for each case. The biopsy slides were reviewed by two pathologists who were blinded with regard to cancer outcome. Results. Women with atypical hyperplasia (AH) had 4.3 times the breast cancer risk of women without PD (95% confidence interval [CI], 1.7–11). In women with PD lacking AH, the relative risk was 1.3 (95% CI, 0.77–2.2). A family history of breast cancer (FH) increased breast cancer risk 2.4 times (95% CI, 1.4–4.3). The joint occurrence of FH and AH had a strong synergistic effect on breast cancer risk. Conclusions. AH is a reliable marker of increased breast cancer risk among women undergoing breast biopsy.

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