Nearly all studies have suggested that the use of oral contraceptives (OC) is not associated with the aggregate risk of breast cancer diagnosed in women aged 20-54. Because of age-specific differences in the breast cancer-parity relationship and because of age-specific differences in other breast cancer risk factors, the Centers for Disease Control reexamined data from the Cancer and Steroid Hormone Study to assess whether OC use has different effects on the risk of breast cancer at different ages of diagnosis. This was a population-based case-control study conducted in eight geographic areas in the United States during 1980-1982. In these data, the relationship between the risk of breast cancer and OC use appeared to vary by age at diagnosis. Among women aged 20-34 years at diagnosis or interview, those who had ever used OC had a slightly increased risk of breast cancer (odds ratio 1.4, 95% confidence interval 1.0-2.1) when compared with women of the same ages who had never used OC. Among these women, there were no trends of increasing or decreasing risk with any measure of OC use. Among women aged 35-44 years, there was no association between OC use and breast cancer. Among women aged 45-54 years, those who used OC had a slightly decreased risk of breast cancer (odds ratio 0.9, 95% confidence interval 0.8-1.0). Among these women, the risk estimates decreased significantly with increasing time since first and last use. Although the slightly increased risk estimates for the youngest women are compatible with findings by other investigators, the decreased risk estimates for the oldest women have not been described in as many studies. Available data provide no reasons for changes in prescribing practices or in the use of OC as related to breast cancer risk.