Background/Aims: In 80% of the patients, Acute pancreatitis (AP) occurs as a self-limiting disease that does not require any specifictreatment; however, in 20% of the cases it occurs in its clinically severe form that may lead to local or systemic complications. The aimof this prospective study was to examine the relationship between the neutrophil to lymphocyte ratio (NLR) and the systemic complications and severity of AP.Materials and Methods: This prospective study included 100 patients with AP. Age, sex, NLR, Ranson scores and the revised Atlantaclassification of the patients were recorded. The patients were divided into two groups according to the Ranson scores as mild andsevere AP. According to the Revised Atlanta classification, the patients were divided into two groups as mild and moderate+severe AP.Results: According to the Ranson score, NLR at the time of admission and at the 48th hour in the severe group was found to be statistically higher than the mild AP group (p<0.01). The receiver operating characteristic (ROC) curve analysis was performed to determinethe cut-off value of NLR at the emergency department in order for it to be used for distinguishing AP patients with and without systemiccomplications. The area under the ROC curve was 0.81. Sensitivity and specificity were 87.50% and 69.05%, respectively, when the NLRcut-off value was >7.13.Conclusion: Neutrophil to lymphocyte ratio is associated with severe AP. We also regard NLR as a valuable parameter for predicting thedevelopment of systemic complications in patients with AP.