PIP: This article reviews the results of the main clinical trials of RU 486 for termination of pregnancy. The radical substituted in position 11 of the molecule gives it its particular properties of fixing to progesterone and glucocorticoid receptors (and to a lesser degree to androgen receptors), thereby blocking the cellular actions of these hormones. Most trials of RU 486 have taken place in France, although some of them have been sponsored by large international organizations. Studies between 1982-85 of RU 486 alone or in combination with an oxytocic agent indicated that the effective dose varied from 400-800 mg. Serious complications were exceptional, although 1 case of septicemia occurred which was cured after treatment for retention of the products of conception. After the 1st trials, studies were done on a larger scale in 1985-86 to determine whether a larger dose administered once or repeated only once would be more effective in termination of pregnancy and causing expulsion of the fetus. Rates of complete success with doses of 400, 450, or 600 mg of RU 486 administered once or twice to terminate pregnancies of 5-7 weeks were higher than with the smaller repeated doses of the 1st trials, but complete or partial failures continued to occur, requiring uterine aspiration. Analysis of results indicated that efficacy was greater with earlier pregnancies. Another study demonstrated that 600 mg RU 486 administered in 1 dose completely interrupted pregnancies of less than 5 weeks in 90% of cases. Doses of 800 mg do not seem to improve results, perhaps because of hepatic metabolism. Another route of administration may improve results. Among 154 women treated with 450 mg of RU 486 in a single dose or with a single repetition 48 hours later to induce an abortion within the 1st 8 weeks of amenorrhea, 97% had uterine bleeding. In 13.2% of cases it was subjectively considered minimal, and in 43.4% eah it was considered moderate or heavy. Bleeding was considered less than in a normal period in 22.1% of cases, equal in 23.5%, heavier in 36.0%, and much heavier in 18.4%. The average delay to appearance of bleeding was 2.6 + or - 1.7 days, and average delay to expulsion was 4.6 + or - 3.0 days. The average duration of bleeding was 4.5 + or - 3.7 days. 43% of women had bleeding of 1-4 days, 32% had 5-8 days, 13% for 9-12 days, and 6% for more than 13 days. It is evident that use of RU 486 as an abortifacient will require strict medical follow-up, so that largescale use will not lead to demedicalization of induced abortion. Nausea and asthenia were initially reported from treatment with RU 486 to induce abortion but were probably due to the pregnancy itself since thay havenot been observed in use of RU 486 with nonpregnant patients. Abdominal and pelvic pain were less intense than those caused by abortifacient use of prostagladins. No sign of glucocorticoid insufficiency has been observed. The product appears to be well tolerated. The most promising uses of RU 486 for the future appear to be to terminate pregnancies within 1 week of a missed period or as a "pharmacologic laminaria tent" to dilate the cervix for uterine aspiration, curettage, or other procedures.