Progesterone and human chorionic gonadotrophin (HCG) in serum and pregnandiol in urine were measured in 64 patients admitted to hospital because of threatened abortion. Blood samples were taken and urine specimens collected at regular intervals during admission and after discharge during the rest of the pregnancy. A reference range was worked out for each hormone based on the hormone values obtained from the pregnancies proceeding to term. The predictive significance of values within and below the reference range was determined, for the initial sample and for serial samples. An association between hormone levels and outcome of pregnancy was observed but it is concluded that both single and serial determinations of progesterone and pregnandiol and serial determinations of HCG are unsatisfactory for the evaluation of threatened abortion. However, an initial progesterones value below the reference range and HCG values below 10,000 mIU/ml between the 8th and 15th week of pregnancy was in every case always followed by spontaneous abortion. A hormonal test of fetoplacental origin is recommended for monitoring threatened abortion.