To investigate the effect of individual formulations of oral contraceptives on the risk of breast cancer in women, we analyzed case-control data from the Cancer and Steroid Hormone Study of the Centers for Disease Control. The cases were 4711 women 20 to 54 years old with newly diagnosed breast cancer who were selected from eight population-based cancer registries. The controls were 4676 women selected by random-digit dialing of the population of each area covered by a registry. As compared with women who had never used oral contraceptives, women who had used them had a relative risk of breast cancer of 1.0. Among women who used only one oral-contraceptive formulation, this estimate of relative risk did not change appreciably according to the formulation used. Neither the type of estrogen nor the type of progestin contained in oral contraceptives used was associated with an increased risk of breast cancer. The duration of oral-contraceptive use and the time since last use did not influence the risk. These findings provide further support for the contention that oral-contraceptive use does not increase the risk of breast cancer in women.