The prognostic value of maternal plasma human chorionic gonadotrophin (hCG) determinations, measured by a specific beta-subunit radioimmunoassay, was studied in 188 patients with bleeding between 6 and 20 weeks gestation. The patients were arranged into different subgroups according to ultrasound findings as well as clinical and histopathological evidence. In threatened abortion with successful outcome (50 per cent of all patients studied), the weekly mean hCG values were normal or even slightly elevated. In patients with a blighted ovum, the first hCG level measured was in the normal range in 34 per cent of the patients. In patients who aborted embryo with former life signs hCG values were generally normal before the abortion. The hCG levels were usually low in patients with incomplete abortion and ectopic pregnancy. An initially subnormal level of hCG was associated with a poor outcome of pregnancy in 92 per cent of patients. If the first hCG level was within the normal range the outcome of pregnancy was favourable in 79 per cent of patients.