RU486, a potent antiprogesterone steroid was administered to 124 women requesting therapeutic abortion. All were less than 49 days from their last menstrual period. Ten of these subjects (Group I) received high doses of RU486 in a decremental dose regimen (400, 300, 200 and 100 mg/day) over 4 successive days and 14 received 50 mg/day for 7 days (Group II). A further 50 subjects (Group III) received 100 mg/day for seven days and the remaining 50 subjects (Group IV) received 450 mg in a single dose. In the first three groups, half the daily dose was given in the morning and the remainder in the evening. Blood was collected before, and on Days 4 and 7 and then once a week after commencing therapy until disappearance of circulating beta HCG. In addition to beta HCG, estradiol-17 beta (E2), progesterone (P), cortisol, and various metabolic and hematological parameters were measured. Plasma RU486 concentrations were also assayed in Group II, III and IV subjects on Day 7 of therapy and in some cases on Days 14 and 21. Ultrasonography was performed in all cases on Day 1 and on Day 14. All the patients bled within five days following RU486 administration, for 1 to 21 days. A complete abortion occurred in 60% in Group I, 50% in Group II, 86% in Group III, and 80% in Group IV. The difference between the last two groups and the first two was significant at p less than 0.01. The non-responders were submitted to a uterine vacuum aspiration. A stepwise discriminant analysis was performed and indicated that the best predictors of the outcome of therapy were beta HCG values and the gestational sac diameter. With these criteria, the prediction was accurate in 86.4% of the cases. The best results were obtained in the cases where the ultrasonic measurement of gestational sac was under 10 mm in diameter and the initial beta HCG values under 15,000 mIU/ml. Among the observed side effects were moderate pelvic cramps (20.9%), nausea (27%), fainting (4.8%); 61.3% of the women complained of fatigue. Heavy bleeding occurred in 15.3% of the women but only one of them required blood transfusion. In the patients with complete abortion, beta HCG values decreased to below 500 mIU/ml by Day 14 (but in 11 cases values fell below 2,000 mIU/ml only by Day 21). Plasma estradiol and progesterone also fell. Cortisol levels increased during therapy especially in subjects of Group I, but returned to basal values after termination of treatment.(ABSTRACT TRUNCATED AT 400 WORDS)