Objective : To obtain information on the variation of Mllerian Inhibiting Substance (MIS) levels according to gestational age and to understand the physiologic effects of MIS during pregnancy. Methods : We measured MIS levels in 325 serum samples from pregnant women and in 59 serum samples from nonpregnant women by ELISA. This study was also designed to enlighten the possibility for early diagnosis of genetic diseases by discriminate the fetal sex at early stage of pregnancy by measuring maternal MIS serum level. Results : These data showed that the mean MIS serum concentration plus or minus standard error was 2.66±0.12 ng/ml in pregnant women, which was significantly higher than 2.05±0.29 ng/ml in non-pregnant women (P<0.05). The median MIS level for pregnant women (2.2 ng/ml) was approximately two-fold greater than that for non-pregnant women (1.2 ng/ml). There was significant negative relationship between MIS concentration and gestational age (r2=0.0434, P<0.01). The MIS concentration of the pregnant women decreased 0.0069 ng/ml per day as gestational age increased. There was no significant difference in MIS levels for women pregnant with males (2.75±0.17 ng/ml) versus females (2.43±0.19 ng/ml). Conclusion : The MIS may play an important role as a inhibiting hormone of quiescent ovary during pregnancy. These data will enable normal and abnormal levels of MIS during pregnancy to be differentiated with higher precision and will facilitate the clinical application of MIS determination as a tumor marker for selected gonadal tumors during pregnancy.