Background. The role of free oxygen radicals in inflammatory conditions is well known. Free radicals cause lipid peroxidation of cellular membranes resulting in cell death. The purpose of this study was to investigate the levels of total anti-oxidant status (TAS), as a marker of anti-oxidant defense system and malondialdehyde (MDA), as a marker of oxidative stress, in the plasma of patients with acute appendicitis. Methods. Fifty-one adult patients with a median age of 31 years who underwent operations with a preoperative diagnosis of acute appendicitis were included in this prospective study. Blood samples for C-reactive protein (CRP), MDA and TAS were collected preoperatively. Groups were compared by using the Mann-Whitney U test. Results. There were 27 patients with acute phlagmenous appendicitis and 19 patients with advanced appendicitis (10 gangrenous and 9 perforated appendicitis), while 5 negative explorations were documented. No significant differences in WBC counts and MDA levels between groups were encountered. Plasma CRP was significantly higher in patients with perforated appendicitis, but not in the other groups. In advanced appendicitis group, TAS level was significantly lower than the other groups. On the other hand, plasma TAS level in acute phlagmenous appendicitis group was significantly higher. Conclusion. A decrease in plasma total anti-oxidant capacity might be a predictor of the progression of inflammation to the perforation in acute appendicitis. © 2006 Ozdogan et al; licensee BioMed Central Ltd.