We performed a prospective study to determine the prevalence and significance of subchorionic hematomas in patients with symptoms of threatened abortion. The study comprised 342 pregnant women who had vaginal bleeding in weeks 9-20 of pregnancy and a live fetus shown with sonography. Sonograms showed a subchorionic hematoma in 62 patients (18%). The average size of the hematoma was 20 ml (range, 2-150 ml). The rate of spontaneous abortion was the same in patients with and without hematoma, seven (11%) of 62 and 28 (10%) of 280, respectively. There was no association between abortion rate and hematoma size. The rate of premature delivery was the same in patients with and without hematoma, seven (11%) of 62 and 32 (11%) of 280, respectively. There was no association between the rate of premature delivery and hematoma size. Subchorionic hematomas are common and insignificant sonographic findings in patients with vaginal bleeding in weeks 9-20 of pregnancy.