The prognostic predictive value of maternal serum CA125 measurement was investigated in 25 cases of threatened abortion. The women were non-smoker, had a ultrasonographically verified viable single fetus, and the gestational ages ranged from 7 to 12 weeks. Twenty-five healty pregnant women, with the same characteristics were used as the control group. The overall abortion rate was found to be 20% (5/25) in the study group. In serial measurements the mean serum CA125 level of the patients with an unfavorable pregnancy outcome was significantly higher than that of the patients with a favorable outcome. When the cut-off level of maternal serum CA125 was taken as > 65 U/ml in the first and > 60 U/ml in the second measurements of the study group, the risk of termination of the pregnancy by spontaneous abortion was 83.3% in the patients with elevated serum CA125 levels. No statistically significant difference was observed with respect to the duration of vaginal bleeding between the aborters and the patients with a favorable outcome. Nevertheless, when vaginal bleeding had been present for 3 days or more and there was high maternal serum CA125 activity, the abortion risk was found to be 100% (3/3). These findings suggest that the maternal serum CA125 measurement in threatened abortion can be useful to determine the extent of decidual destruction which is directly related to the outcome of pregnancy.