Epidemiologic studies indicate that the risk of complications with oral contraceptive use is related to the steroid content and potency of the various formulations. This paper summarizes human data in which potencies of progestins in oral contraceptives can be compared. Data on delay of menses and endometrial subnuclear vacuolization, an indirect assessment of glycogen deposition, are presented. The relative effects of various progestins on serum lipids and lipoproteins are also summarized. The object of this review is to examine the available scientific evidence which generally supports the conclusion that there is a marked similarity of potency of the dose of various progestins used in many of the formulations currently available in the U.S. The progestins norethindrone, norethindrone acetate and ethynodiol diacetate are roughly equivalent in potency while norgestrel is roughly five to ten times and levonorgestrel ten to 20 times as potent.