Using oral contraceptives for long periods or using them when one has other risk factors has been hypothesized to increase the risk of breast cancer. To study these issues, we analyzed data from a multicenter, case-control investigation—the Centers for Disease Control's Cancer and Steroid Hormone Study. All women 20 to 54 years old with a first diagnosis of breast cancer ascertained by eight population-based cancer registries are study subjects; controls are selected at random from the general population of these eight areas. Analysis of the first 689 cases and 1,077 controls studied showed that women who had used oral contraceptives at some time in their lives had a relative risk of 0.9 compared with never-users (95% confidence interval, 0.8 to 1.2). Neither duration of oral contraceptive use nor time since first use altered a user's risk of breast cancer; women whose first use was more than 15 years ago and who used oral contraceptives for 11 years or more had a relative risk of 0.8 (0.5 to 1.4). Oral contraceptive use did not increase the risk of breast cancer among women with benign breast disease or a family history of breast cancer. Oral contraceptive use before a woman's first pregnancy did not increase her risk of breast cancer significantly more than other methods of delaying first pregnancy. This study provides no support to the hypothesis that oral contraceptive use increases a woman's risk of breast cancer.