Background: Copeptin is a stable by-product of the syn-thesis of arginine-vasopressin (AVP); it can be accurately and quantitatively measured in plasma and mimics the re-lease of AVP.    
Aim of work: to assess the importance of maternal serum copeptin levels in diagnosis of threatened preterm labor and prediction of preterm birth.  
Subjects & methods: This study was carried out as a pro-spective case control on 88 pregnant women at Obstet-rics and Gynecology Department, Faculty of Medicine, Menoufia University. They were divided into two groups: Group I (Case group n=58): including 58 pregnant la-dies who admitted for threatened preterm labor.  Group II (Control group n=30): including 30 healthy pregnant ladies of matched age, parity, and gestational age.   
Results: Serum copeptin was significantly higher in cas-es with preterm labor than term controls (641.29±167.87 pg/ml vs 192.67±71.16 pg/ml) respectively. Regard-ing the outcome serum copeptin was significantly high-er in cases with preterm birth than cases with term birth (751.57±169.923 pg/ml vs 642±163pg/dl) respectively. The serum copeptin cutoff value is ≥ 380 pg/ml, the area under the curve is equal to 1.00, according to our ROC data. The sensitivity, specificity, positive predictive and negative predictive values of serum copeptin were 98.3%, 100%, 100% and 96.8% respectively.  
Conclusion: Maternal serum copeptin levels can serve as a valuable diagnostic and predictive marker in cases of threatened preterm labor and preterm birth.