Introduction: Walled-off pancreatic necrosis (WOPN) is a life-threatening, late complication of acute pancreatitis, in which a fluid collection containing necrotic material is formed. Infection of the fluid collection significantly increases the mortality of patients with WOPN.Aim: To examine the levels of oxidative stress markers in the pancreatic necrotic fluid (PNF) and serum of patients with sterile and infected WOPN.Material and methods: Thirty-three adult patients with sterile WOPN and 14 with infected WOPN, as well as 31 patients with mild AP, were included in this study. Concentrations of oxidative stress markers (8-isoprostane, protein carbonyl groups, and 8-hydroxyguanine) were measured in the PNF and serum of patients with sterile and infected WOPN.Results: High concentrations of all measured oxidative stress markers in PNF, but not in serum, were detected in patients with WOPN. Additionally, oxidative stress markers in PNF were significantly increased in patients with infected as compared to sterile WOPN. The serum high sensitive C-reactive protein (hsCRP) concentrations showed the highest correlation with PNF oxidative stress marker levels. Receiver operating characteristics (ROC) curve analysis confirmed that serum hsCRP could be a good predictor of WOPN infection.Conclusions: Oxidative stress is associated with WOPN development; infection of PNF worsens the course of WOPN, possibly via increased production of reactive oxygen species; and serum hsCRP concentrations seem to be a good, noninvasive indicator of PNF infection.