Background: Adenosine deaminase (ADA) is found in most tissues including lymphoid cells and lymph nodes. It is a marker of T lymphocyte activation. The role of type 1 and type 2 T helper cells in appendicitis has been investigated experimentally. Serum ADA levels in acute appendicitis have not previously been studied. Aim: To assess the serum levels of ADA in patients with acute appendicitis. Methods: Serum levels of ADA were investigated in 30 cases with acute appendicitis (mean age 26 years; male/female 17/13) and 21 healthy controls (mean age 40 years; male/female 11/10). Levels of ADA were compared in patients with acute appendicitis and healthy controls. Correlation analysis between ADA and other inflammatory markers (C-reactive protein ICU), high-sensitivity CRP, erythrocyte sedimentation rate and white blood cell count) was also performed. Results: Mean (SD) serum ADA levels were significantly higher in those with acute appendicitis than in the control group (13.41 (3.56) U/I vs 9.39 (1.22) U/I; p<0.001). There was no correlation between ADA and the other inflammatory markers investigated. Conclusions: Although serum levels of ADA do not correlate with other known inflammatory markers, its serum level is increased in acute appendicitis and it has a higher positive predictive value.