Introduction & Objective: The aim of this study is to evaluate the value of elevated white cell count (WCC), C-Reactive Protein (CRP) and bilirubin in the diagnosis of acute appendicitis. Materials & Methods: In this prospective study, all patients (n=189), with the diagnosis of acute appendicitis, who had undergone operation in teaching hospitals during one-year period, were studied, regardless of their age. After considering the exclusion criteria, 149 patients were evaluated. In all patients, in addition to routine tests, measurement of C-reactive protein (CRP) and bilirubin were performed. Surgery was performed on the basis of Alvarado score. After surgery, the patients were classified into three groups (positive appendectomy, negative appendectomy and perforated appendicitis) according to pathological findings, and the results were evaluated. Results: Out of 149 patients, 86 were males and were 60 females with e mean age of 28.1±12.7 years. The pathological examination revealed that 20 patients (13.4%) had negative appendectomy, and Positive appendectomy was observed in 129 patients (6.86%), of which 23 had perforated appendicitis that was confirmed by histopathologic results. In this study, a statistically significant difference was observed between the patients with positive and patients with negative appendectomy, in terms of levels of CRP, mean total WCC and mean levels of bilirubin (P