A prior report suggested that active-duty pregnant women are at increased risk for low-birth weight infants and a higher perinatal mortality rate. The present double-blind investigation was designed to prospectively evaluate that risk and to test the efficacy of 17 α-hydroxyprogesterone caproate to prevent reported complications. Three groups of active-duty women were studied, beginning between 16 and 20 weeks' gestation. They were similar for parity, previous abortion, race, cigarette smoking, and marital status. Of these, 80 were given 17 α-hydroxyprogesterone caproate, 88 received placebo, and 78 declined to participate in the protocol. There was no significant difference in the three groups when comparisons were made for low-birth weight infants and for perinatal mortality. However, when comparison was made to a military dependent population, they had a significantly worse outcome with regard to both perinatal mortality (p = 0.001) and infants with a birth weight < 2,500 gm (p = 0.01). We concluded that pregnant military personnel were at increased risk for adverse pregnancy outcome, but that this risk was not altered by therapy with 17 α-hydroxyprogesterone caproate.