Abstract Objective: The relationship between adenosine deaminase activity and plasma adenosine was examined in non-pregnancy as well as in singleton and twin pregnancies. Methods: Whole-blood samples were taken from 11 singleton and 11 dichorionic twin pregnant women during the third trimester. The plasma adenosine levels were assayed with a modified high-performance liquid chromatographic method, while the adenosine deaminase levels were measured enzymatically. Results: The adenosine deaminase and plasma adenosine levels were both higher in twin pregnancies compared to singleton and non-pregnancies. Conclusion: Increased plasma adenosine is not due to decreasesd serum adenosine deaminase levels in twin pregnant women.