Eighty pregnant women at high risk of giving birth prematurely were divided randomly into two groups. Treatment with either 17 alpha-hydroxyprogesterone caproate, 250 mg by intramuscular injection once a week, or a placebo was given in a double-blind fashion. Imminent premature labor occurred in 29.0% of the treated group and in 59.4% of the control group (p less than 0.025). The rate of premature deliveries was also significantly lower in the treated group (16.1%) than in the control group (37.82%) (p less than 0.05). There were no cases of perinatal death or fetal malformations in either group. The mean birth weight of all infants of the treated group was significantly higher than in those of the control group (3111.9 +/- 905 gm versus 2680 +/- 813.4 gm, p less than 0.05). The results support treatment with progesterone caproate for the prevention of premature labor.