An immunoradiometric assay for an ovarian tumor associated antigen (CA 125) has recently been developed. The CA 125 assay was reported to be fairly sensitive for ovarian cancer. However, elevated CA 125 levels were reportedly observed in 1% of healthy controls, 6% of patients with miscellaneous diseases and 29% of patients with non-gynecological cancer. Therefore, the specificity of the CA 125 assay must be evaluated before its wide clinical application. Furthermore, data on the CA 125 levels in pregnant women have not heretofore been reported. In the present study, CA 125 levels were determined in sera randomly obtained from normal, pregnant women at various weeks of pregnancy and in sera serially collected from women who achieved pregnancy after ovulation induction. When compared with the normal, non-pregnant state, CA 125 levels in pregnant women were significantly higher during the first trimester, but not during the second and third trimesters. Although CA 125 levels in women during the first trimester of pregnancy were significantly lower than those in patients with ovarian cancer, there was a considerable overlap between them. CA 125 levels were undetectable before pregnancy in all of the 8 patients who achieved pregnancy after ovulation induction. They rose during the first trimester of pregnancy in 7 of the 8 patients, and they decreased thereafter to undetectable levels by the end of the first trimester. The increased CA 125 level in the early pregnancy provides an important information for the clinical application of the CA 125 assay. Pregnancy should be ruled out when increased CA 125 levels are found in women during the childbearing years.