Adenosine deaminase (ADA) is suggested to be an important enzyme for modulating the bioactivity of insulin, but its clinical significance in diabetes mellitus (DM) is not yet characterized. We measured the serum level of ADA in healthy controls (C, n=29) and type 2 diabetic patients (n=42). The mean serum level of ADA in C, and type 2 diabetic patients was 13.04 +/- 3.3 and was 22.2 +/- 4.3 U/L, respectively (P<0.01[corrected] vs. C). ADA levels of patients were significantly correlated with HbA1c (r=0.45, p<0.01). Our findings suggest that ADA may play a role in insulin effect and glycamic control. On the other hand, increased activity of ADA in type 2 DM might be a marker for insulin indication. However, further studies are required for the pathogenic role of elevated ADA activity in type 2 DM.