OBJECTIVE The purpose of this study was to analyze the possible usefulness of ultrasonography for the diagnosis of acute appendicitis in children and to compare this technique with the clinical signs classically employed in the evaluation of pain in the right lower quadrant (RLQ) in the emergency ward. METHODS We prospectively analyzed 112 patients younger than 14 years with suspected appendicitis. After a careful clinical record and physical and complementary studies were performed, we performed a ultrasonography of the RLQ on all patients a tubular, non-mobile, non-compressible image with a target image measuring 6 mm or more on the cut section was considered suggestive of appendicitis. After the initial clinical-radiological evaluation, the patients were either operated or included in a follow-up group. None of them were dismissed. The confirmation of the diagnosis of appendicitis was histological. RESULTS Only 4 of the 14 factors analyzed showed a significant association with acute appendicitis (leukocytosis, left shift, abdominal RX film suggesting inflammation in the RLQ and ultrasonography positive for appendicitis), with ultrasonography being the technique with the highest diagnostic reliability (77.7% sensitivity and 94.8% specificity). The age of the patients with appendicitis was significantly lower than that of the patients without appendicitis. Nevertheless, we did not find any relationship between advanced appendicitis and factors such as age or time of evolution. CONCLUSIONS After a clinical-ultrasonography evaluation, our negative appendectomy rate was 7% and the perforation/gangrenous appendicitis rate was 29%. We conclude that ultrasonography performed by trained professionals is a useful tool in the differential diagnosis of appendicitis in children.