During a 4-month period, high-resolution ultrasonography (US) was used to prospectively evaluate 70 children with clinically suspected acute appendicitis. Thirty-five US scans showed a noncompressible appendix with maximal outer diameters greater than 6 mm. This finding was considered positive for the diagnosis of acute appendicitis. Thirty-one of these 35 patients had acute appendicitis documented by surgical and pathologic findings. The remaining four patients were observed, and their symptoms resolved. Thirty-five patients had US scans considered negative for appendicitis. Seventeen of these patients had US findings positive for other conditions including mesenteric adenitis, ileitis, intussusception, Crohn disease, and Burkitt lymphoma. In this series US enabled diagnosis of acute appendicitis with a sensitivity of 94%, a specificity of 89%, and a predictive accuracy of 91%. Diagnosis of acute appendicitis can be made with US with the same accuracy in children as has been previously reported in series of adult patients. The use of US in clinically ambiguous cases may allow earlier diagnosis, prevention of perforation, and decreased complications in the pediatric patient with acute appendicitis.