A number of synthetic progestogens are currently available which differ greatly among themselves in various ways. The common property of all progestogens is that they transform a proliferative endometrium into a secretory or luteal endometrium by fixing the progestogen or 1 of its metabolites to the progesterone receptor. Most progestogens also have a greater or lesser affinity for other hormonal receptors, and some cause modifications in metabolism, especially of lipids and glucose. Synthetic progestogens can be classified according to their chemical formulas, biologic properties, and efficiency in relation to hormone receptors, but none of the current classification systems is a satisfactory guide to use. It is not yet definitively known whether pro-hormones, which must be transformed into norethindrone in vivo before taking effect, are advantageous or disadvantageous for therapeutic use. Synthetic progestogens have been found to have varying metabolic effects according to their content and dosage; hepatic function, lipid metabolism, glucose metabolism, coagulation factors, and the renin-angiotensin-aldosterone system are among the functions affected. The metabolic effects of synthetic progestogens are the principal criteria of choice. High dose 19 norsteroids are recommended only for cancer treatment, 19 nor-pregnane derivatives and progesterone isomers appear suitable for treatment of conditions such as endometriosis, premenopausal menstrual irregularities, and menstrual irregularity resulting from luteal insufficiency in younger women. Low-dose 19 norsteroids remain the best choice for contraception. Levonorgestrel has been preferred over norethindrone for some time because it is effective at a dose of .150 mcg compared to 1 mg for norethindrone, but some recent research suggests that even at a much smaller dose, levonorgestrel may cause more metabolic modifications than norethindrone. The pro-hormones ethynodial diacetate and lynestrel have additional metabolic effects whose consequences are as yet unknown. Preparations containing levonorgestrel should be preferred until the expected appearance on the market of triphasic preparations containing norethindrone and desogestrel, which permit excellent cycle control. Among progestogen-only contraceptives, 10 mg/daily of lynestrenol is associated with high rate of side effects. Microdose synthetic progestogen preparations are sometimes useful but offer imperfect efficacy, poor cycle control, higher risk of extrauterine pregnancy and ovarian cysts, and creation of iatrogenic luteal insufficiency.