A case-control interview study, conducted among participants in the Breast Cancer Detection Demonstration Project and involving 963 breast cancer cases and 858 controls, allowed evaluation of the risk of breast cancer associated with use of oral contraceptives. Overall, there was no association between use and risk of disease (RR = 1.1). In addition, there was no indication of increasing risk with years of use or years since initial use, despite slight excess risks observed among users of high-dose preparations. Premenopausal women who used the pill after the age of 40 demonstrated approximately a 50% increased risk, possibly as a result of artificial prolongation of a premenopausal rate of disease incidence. Non-significant excess risks associated with pill use were also seen among premenopausal women who reported a family history of breast cancer in a sister (RR = 3.6) or previous biopsies for benign breast disease (RR = 3.2). The latter excess was limited to women whose use of the pill preceded a first biopsy, suggesting that the types of lesions requiring biopsy among current long-term pill users may be those that predispose to breast cancer.