The clinical and endocrine effects of progestogen therapy in early pregnancy were investigated using a double-blind randomized trial in 64 patients who had a viable fetus at 6 weeks gestation and had an increased risk of miscarriage. The patients were randomly allocated to receive either 17 alpha-hydroxyprogesterone caproate or a placebo between 7 and 12 weeks gestation. Four fetal ultrasonographic variables and 17 maternal endocrine variables were studied in each woman. Only four maternal serum variables (17 alpha-hydroxyprogesterone, prolactin, thyroxin and thyroxin binding globulin) rose significantly. The serum progesterone levels in the hormone supplemented group were on average 20% higher than in the placebo group but the difference was not statistically significant. However, the relation between the progesterone levels and the fetal outcome was not clear. Therefore it is not advisable to prescribe 17-OHP-C during early pregnancy to prevent a miscarriage.