Fourteen women experiencing early pregnancy failure have been studied during the time of conception and at frequent intervals until spontaneous abortion occurred. Serial measurements of serum estradiol, progesterone, 17 alpha-hydroxyprogesterone, prolactin, human placental lactogen (hPL), and human chorionic gonadotropin (hCG) were determined; regular sonar scanning allowed the time of fetal death to be determined to within 7 days in six patients and a diagnosis of blighted ovum to be made in the remainder. In all patients serum progesterone and estradiol concentrations were within the normal range up to 7 weeks but appeared to decrease from about 8 weeks' gestation whether or not a living fetus was present. The placenta continued to produce hCG and hPL but, despite the continuing presence of hCG, the levels of 17 alpha-hydroxyprogesterone declined to concentrations below those associated with normal pregnancy. These data suggest that the placenta may require a particular stimulus to take over production of progesterone and estradiol.