Background. Decision-making for management may sometimes be difficult in acute appendicitis (AA). Various diagnostic scoring systems exist, but their sensitivity and specificity rates are far from ideal. In this study, the determination of the predictors and the effect of radiological data and developing a new scoring system were aimed. Methods. Medical records of patients who were hospitalized for AA between February 2012 and October 2016 were retrospectively reviewed. All data were compared between patients with and without appendicitis. The multivariate analysis was performed to define significant variables and to examine the sensitivity and specificity of each group of predictors including radiological data. A new scoring system (NSS) was formed and was compared with two existing scoring systems: pediatric appendicitis score (PAS) and Alvarado scoring system (ASS) by using reclassification method. Results. Negative appendectomy rate was 11.3%. Statistical analysis identified 21 independently significant variables. The heel drop test had the highest odds ratio. Sensitivity and specificity rates of clinical predictors were 84.6% and 94.8%, respectively. Radiological predictors increased the sensitivity rate to 86.9%. Sensitivity and specificity rates for PAS, ASS, and NSS were 86.8% and 83.9%, 84.7% and 81.6%, and 96.8% and 95.6%, respectively. The "re-assessed negative appendectomy rate" was 6.2% and false positive results were remarkably more common in patients with duration of symptoms less than 24 hours. Conclusion. Radiological data improves the accuracy of diagnosis. Containing detailed clinical and radiological data, NSS performs superiorly to PAS and ASS, regarding sensitivity and specificity without any age limitation. The efficiency of NSS may be enhanced by determining different predictors for different phases of the inflammatory process.