Aim: To measure the serum concentration of CA125 in pregnancies complicated by preeclampsia and low risk pregnancies with the pos- sibility it being considered to be used in early diagnosis of severe preeclampsia so, it may be a marker of the severity of the disease. Methods: One hundred and eighty-nine pregnant women were par- ticipated in the study, it was carried out at department of obstetrics and gynecology, Suez Canal University Hospitals. They were divid- ed into 3 groups mild, severe preeclampsia and normal study group. Urine analysis, complete blood count (CBC), serum AST and ALT, uric acid, urea, creatinine and serum CA125 were assayed for all participants. Results: The mean serum concentration of CA-125 was (32.59 ± 1.63), (39.70± 1.19) and (52.92 ± 2.88) in control, mild and se-  vere preeclampsia respectively, the difference was statistically significant (p=0.001).It wasfoundthatCA125positivelycorrelated- withsystolic blood pressure, diastolic bloodpressure, proteinuria, hemoglobin level,ALT, AST, serum uric acid,serum creatinineand urea,meanwhile, the plateletcountshowed negative correlation with- CA125(p<0.05).Receiver operating characteristics (ROC) curve was used to find out the best cut off value of CA 125 in mild and sev-  er preeclampsia, it was 39 in mild preeclampsiawith sensitivity of 80.9%, specificity 99 %, PPV 99.4% and NPV 84%.The best cut off value was 49.5 in severe preeclampsia, the sensitivity, specificity, PPV and NPV were 82.5 %, 99 %, 99 % and 87.2 respectively. Conclusion: SerumCA-125leveliselevatedsignificantly inmild and severepreeclampsia,itcorrelated with the severity of preeclamp- sia,soserum concentration ofCA125 maybeusedas a marker ofsever- ityof preeclampsia.