Antiprogestins, agents that inhibit the action of progesterone, are among the most controversial and yet the more interesting therapeutic compounds developed in the past 20 years. These agents provide the most effective and safest means of medical abortion, and in addition they may be used for the treatment of patients with cancer, Cushing's syndrome, and gynecologic disorders and for contraception. The first effective antiprogestin was mifepristone (also known as RU 486),¹ a derivative of the progestin norethindrone (Figure 1). By analogy with the antiestrogens, it is likely that the substitution at the 11 beta position is responsible for the antiprogestin . . .