Premenopausal bilateral breast cancer is characterized by a strong family risk, and, consequently, a high probability that inherited susceptibility genes may be segregating in these families. Determining whether risk factors that affect other breast cancer cases have a similar effect in the etiology of bilateral breast cancer is of interest. Therefore, as part of an ongoing genetic-epidemiologic study of premenopausal bilateral breast cancer, we conducted a case-control analysis of reproductive variables, benign breast disease, alcohol, and smoking. Cases had premenopausal bilateral breast cancer, and their unaffected sisters served as controls. A set of reproductive variables--including earlier age at menarche, nulliparity, and late age at first full term pregnancy--appeared to increase the risk of breast cancer; the corresponding confidence limits, however, were wide and straddled the null. In addition, other variables associated with increased premenopausal bilateral breast cancer risk were: use of oral contraceptives, history of benign breast disease, and high alcohol consumption. We found no positive association for smoking. Nulliparity and late age at first full-term pregnancy appeared to have different effects in women with family histories of breast cancer than in women without such a history. We detected no substantial effect modification for the other risk factors. In general, risk factors previously identified for breast cancer (usually postmenopausal, unilateral cases) appear also to increase the risk for premenopausal, bilateral breast cancer.