The present study was designed to estimate the prognosis for patients admitted with threatened abortion on the basis of assays of serum estradiol (E2) and progesterone (P) and the demonstration of fetal heart movements (FHM) by ultrasound scanning. Of the 97 patients included in the study, 35% (with 95% confidence limits 25 to 45%) aborted spontaneously. Of the patients with normal serum E2and P levels, 88% (74 to 96%) and 91% (79 to 98%), respectively, continued to term, whereas 52% (39 to 66%) and 58% (44 to 72%), respectively, with pathologic levels aborted. Ultrasound scanning for FHM exhibited the greatest diagnostic accuracy: 92% of the patients (83 to 97%) with FHM at the first scan carried to term, and 91% (75 to 98%) without FHM aborted. After 9 weeks' gestation, the recording of FHM, if any, was reliable in 100% of the cases. It is concluded that ultrasound scanning with determination of FHM is of greater prognostic value than hormone assays in patients with threatened abortion.