The risk of breast cancer in relation to oral contraceptive use was evaluated in a case-control study of 1191 patients with breast cancer and 5026 control patients. For ever-use compared with never-use, the estimated relative risk of breast cancer was 1.0 (95% confidence interval 0.9-1.2). Use of oral contraceptives for five or more years was not associated with breast cancer, regardless of whether use had ended as much as 10 or more years previously, or more recently. Within categories of women whose baseline risk was elevated, including nulligravidae, premenopausal women, and those with benign breast disease or history of breast cancer in first-degree relatives, the relative risk estimates for five or more years of oral contraceptive use approximated 1.0. For any use before first pregnancy, the relative risk estimate was 1.3, and for use lasting three or more years it was 0.9. These data suggest that long-term oral contraceptive use does not increase the risk of breast cancer even after a latent interval in excess of one decade; nor do oral contraceptives appear to increase the risk within categories of women at relatively high baseline risk.