The physiological effects of progestins are mediated by the progesterone receptor, a member of the steroid/nuclear receptor superfamily. As progesterone is required for maintenance of pregnancy, its receptor has been a target for pharmaceuticals. Here we report the 1.8 A crystal structure of a progesterone-bound ligand-binding domain of the human progesterone receptor. The nature of this structure explains the receptor's selective affinity for progestins and establishes a common mode of recognition of 3-oxy steroids by the cognate receptors. Although the overall fold of the progesterone receptor is similar to that found in related receptors, the progesterone receptor has a quite different mode of dimerization. A hormone-induced stabilization of the carboxy-terminal secondary structure of the ligand-binding domain of the progesterone receptor accounts for the stereochemistry of this distinctive dimer, explains the receptor's characteristic pattern of ligand-dependent protease resistance and its loss of repression, and indicates how the anti-progestin RU486 might work in birth control. The structure also indicates that the analogous 3-keto-steroid receptors may have a similar mechanism of action.