We conducted a double-blind study to determine the efficacy of 17α-hydroxyprogesterone caproate in preventing premature delivery in 43 high-risk patients. Premature delivery did not occur in 18 patients receiving the progestational agent, whereas 41 per cent of the 22 receiving the placebo had premature delivery (P < 0.01). The mean duration of pregnancy and the mean birth weight in the former group (38.6 weeks ± 1.6 S.D., and 2836 g ± 412 S.D.) were both significantly greater (P <0.025) than that in the latter (35.2 weeks ± 6.7 S.D.; 2361 g ± 1085 S.D.). The perinatal mortality rate in the group given the progestational agent (0 per cent) was significantly less than that observed in the placebo group (27 per cent) (P < 0.05). Although there were no complications attributable to the progestational drug, the study population was too small for assessment of immediate or long-term safety. However, the results indicate a possible obstetric use for this drug. (N Engl J Med 293:675–680, 1975)