BACKGROUND: Acute appendicitis (AA) is the most common reason for pediatric abdominal surgery in the world. Despite ad- vances in science and technology, diagnosing AA is still difficult today, and complications are common as a result. The early prediction of complicated appendicitis is of great importance for the surgical planning, further treatments, and predicting the course of disease. The immature granulocyte (IG) is a new and more effective marker in predicting the severity of inflammation than traditional markers. Our aim is to determine the effectiveness of IG% in the diagnosis and severity of AA. METHODS: Eighty-eight patients diagnosed with AA and a control group of fifty-eight healthy children were included in this prospec- tive study. Patients with pathologically confirmed AA were divided into two subgroups: acute simple appendicitis (ASA) and acute perforated appendicitis (APA). The demographic characteristics, white blood cell (WBC) count, neutrophil-to-lymphocyte ratio (NLR), mean platelet volume (MPV), IG%, and C-reactive protein (CRP) values were analyzed. Receiver operating characteristics (ROC) anal- ysis was used to compare the diagnostic accuracies and predictive performances. RESULTS: Patients with AA had higher IG%, WBC count, NLR, and MPV value than control group (p=0.28, p=0.22, p<0.001, p=0.001, respectively). Only IG% showed statistically significant difference from other inflammatory markers evaluated in ASA and APA patients (p<0.001). ROC analysis showed that IG% is a good predictor for the presence of APA at an optimal cut-off for IG being 0.2% (sensitivity 81.8%, specificity 85.2%, area under the ROC curve 0.83). CONCLUSION: In the present study, we demonstrated that AA patients with higher IG levels might be more likely to develop perforation. The IG values combined with a physical examination, imaging studies, and other laboratory tests may help clinicians to identify high-risk AA patients in the pediatric emergency department.