The influences of potential risk factors for benign breast disease were assessed using women twins in a matched pair design. Two groups of cases from the Kaiser-Permanente Twin Registry were considered: 1) 90 pairs of female twins in which one twin reported a history of benign breast disease confirmed by biopsy and her twin reported no history of benign breast disease, and 2) 48 pairs of female twins in which the case had clinically diagnosed fibrocystic benign breast disease and her twin was free of disease at examination and reported no history of the disease. Results were similar in these two samples. A significant positive association was found between benign breast disease and coffee consumption. Oral contraceptive use and greater body mass were inversely associated with benign breast disease after controlling for possible confounding variables by matched-pairs multiple logistic analysis. All associations were stronger for monozygotic than for dizygotic pairs. Twin pairs discordant for disease provide an excellent sample in which to assess the importance of potential risk factors while controlling for early environmental and genetic backgrounds.