Acute appendicitis is an uncommon diagnosis in early childhood. It is frequently complicated by delays in diagnosis, perforation, and prolonged hospital stay. The aim of this study was to evaluate the usefulness of ultrasonography (US) in the management of suspected appendicitis in children younger than 3 years of age. Seventy-one children under age 3 with acute abdomen and possible acute appendicitis, but with atypical findings, were included. All of the children underwent a US scan to rule out appendicitis. There were 41 positive US results for appendicitis. Three children had false-positive scan results and 2 children had false-negative results. Twenty-eight patients had ruptured appendicitis with tumor formation on US. Eighteen of the 28 patients received emergency appendectomy and the other 10 patients received initial conservative treatment with interval appendectomy. Compared with those who received emergency appendectomy, patients who received initial conservative treatment had a significantly smoother postoperative course, including shorter postoperative hospital stay and earlier postoperative oral intake. The overall sensitivity, specificity, accuracy, positive predictive value, and negative predictive value of abdominal US to diagnose acute appendicitis in cases of atypical findings in early childhood are 95.0%, 90.3%, 92.9%, 92.6%, and 93.3%, respectively. US is a useful and highly accurate tool to assess acute appendicitis in early childhood. US has the potential to aid in diagnosis of early childhood appendicitis to avoid the frequent rupture with tumor formation seen at this age. Initial conservative treatment for ruptured appendicitis with tumor formation is safe and is associated with a smoother postoperative course in early childhood.