A case-control study of 452 breast cancer patients, aged less than 50 years, and 872 age-, race-, and religion-matched control patients generated relative risk estimates of breast cancer associated with oral contraceptive practices. The relative risk of breast cancer from ever-use of oral contraceptives was 1.1, not significant. Relative risks did not differ by age, interval since first use, interval since last use, or time periods in which steroid compounds differed in composition and potency. However, the relative risks of breast cancer from current use, from 2 to 4 years of ever-use, from 6 or more years of use by women with prior benign breast disease, and from use before first childbirth were increased significantly. The findings suggest the malignant process may be quickened if transformed cells are present during oral contraceptive use. Yet, the findings neither indicate that oral contraception induces breast cancer nor do they exonerate female steroid hormones. The findings do encourage continued surveillance of steroid contraception for cancer induction or promotion. In addition to duration of oral contraceptive use and other measures of dose response, future observations should pay especial attention to use by women before first childbirth and by women with already established benign breast disease.