Recent studies have yielded conflicting findings on the breast cancer risk posed by oral contraceptive (OC) use, with a retrospective study by Pike, Henderson et al. in Los Angeles suggesting that early and prolonged combined OC use could considerably increase such risk but a largescale inquiry by the Centers for Disease Control finding no effect. An early menopause significantly lowers the risk of breast cancer. OCs, which suppress ovulation in a manner similar to that of menopause, nevertheless appear to lack the protective effect of menopause. A number of studies have found that woen with breast cancer have higher levels of estradiol (E2) not fixed to sex hormone binding globulin than do other women, as would be expected if estrogens are an important etiologic factor in breast cancer. The role of progesterone is suggested by numerous observations of the importance of age at 1st pregnancy, the relationship between the mitotic activity of the mammary tissues and the phases of the menstrual cycle, and the influence of the age at beginning of regular cycles or length of cycles on the risk of breast cancer. Since the peak mitotic activity of the mammary tissues occurs in the luteal phase when levels of estrogen and progesterone are high, it may be that combined OCs also stimulate mitotic activity of the breast tissue. It is difficult to test the hypothesis that OCs with the strongest estrogen and progestin doses have the greatest carcinogenic effect on mammary tissue because the various synthetic hormones differ in type as well as in dose levels, and there is no standardized and generally accepted method of measurement. However, when the different components of combined OCs are classified according to their effect on the endometrium, a positive correlation is observed between their progestational power thus defined and the risk of breast cancer.