Background: The mechanisms involved in the pathology of acute appendicitis (AA) and the factors affecting the progression have still been investigated. Oxidative stress is one of the factors of interest. Nitric oxide (NO) and its role in AA has not been studied previously. Methods: Thirty-four patients who underwent operation with a perioperative diagnosis of AA and 16 age and sex-matched controls were included in the study. Serum thiobarbituric acid reactive substances (TBARS), thiol groups (SH), NO metabolites (NOx), and conventional inflammation markers were determined. Results: NOx, TBARS, C-reactive protein (CRP) levels, white blood cell (WBC) count, and erythrocyte sedimentation rate (ESR) were significantly higher, and total SH was significantly lower in AA than in control group. NOx, TBARS, and SH levels were comparable in acute phlegmonous appendicitis and advanced appendicitis. There was a significant positive correlation between NO x and TBARS, CRP, ESR, WBC and a significant negative correlation between NOx and SH. Conclusions: Serum NOx levels and oxidative stress elevate in AA independent from the extent of the lesion. Increased NO may play a role in the increased oxidative stress in AA. © 2010 Wiley-Liss, Inc.