We estimated the effect of oral contraceptive (OC) use on premenopausal bilateral breast cancer in a matched case-control study. One hundred forty-four cases were identified from population-based registries of Los Angeles County, California, and of Connecticut and from the major hospitals in Montreal and Quebec City. Matched controls were the unaffected sisters of the cases. When age was included in the model, ever-use of OCs for 1 year or more was associated with an odds ratio 1.7 (95% confidence interval = 1.0-2.9). The odds ratios associated with 1-2, 3-6, and 7 years of use were 1.2 (0.61-2.4), 2.5 (1.2-5.3), and 2.0 (0.93-4.2), respectively. Too few women had used OCs before their first full-term pregnancy or before age 25 for these estimates to be informative. Restricting the analyses to women who had ever given birth yielded an odds ratio for ever-use of OCs of 2.1 (1.0-4.4). The results indicate an increased risk of premenopausal bilateral breast cancer associated with OC use.