Earlier work suggested that high levels of CA125 could be detected in the sera of women with threatened abortion. In this study, we used CA125 as a method of anticipating outcome in cases of threatened abortion and compared the results with ultrasonic findings. Three groups of patients were studied.(1) 57 women who threatened to abort, but whose pregnancy continued.(2) 43 women with a threatened abortion, who subsequently aborted, and (3) 50 normal women who had no bleeding in early pregnancy. There was a highly significant increase in serum CA125 in women who aborted compared with the other two groups (P<0.001). The results compared well with conventional ultrasound and serum CA125 may be developed as a cheap, sensitive and specific predictor of outcome in cases of threatened abortion.