Background The purpose of this study was to evaluate the efficacy of ultrasound (US) for the diagnosis of acute appendicitis in children. Patients and methods We reviewed all sonograms for acute appendicitis in children suspected of having appendicitis from January 2009 to December 2012. Sonographic findings were positive when the largest axial diameters were >6 mm. The sonograms that were unable to find the appendix were classified into an other group and were considered separately. Sonographic findings were compared with surgical pathological findings or discharge diagnoses in cases of no surgery. Subgroup analysis in the surgical group, including patient age ( Results The appendix was identified in 270/428 cases (63.1%). The overall sensitivity was 71.2% [95% confidence interval (CI): 56.9–82.9%], specificity 97.7% (95% CI: 94.7–99.3%), positive predictive value 88.1% (95% CI: 74.4–96.0%), negative predictive value 93.4% (95% CI: 89.4–96.3%), accuracy 92.6%, and receiver operating characteristic = 0.84. The specificity (47.1%), negative predictive value (61.5%), and accuracy (76.3%) were significantly lower in the surgical group. The efficacy of US between the sexes and age groups showed no significant difference, except the lower positive predictive values in the younger age group. Conclusion US efficacy for the diagnosis of appendicitis in children is high enough to use as an imaging modality of first choice to reduce complications, hospital stay, and negative appendectomy rate.