Diagnosis of acute appendicitis in children is sometimes difficult. The aim of this study is to validate a clinical scoring system and ultrasonography for the early diagnosis and treatment of appendicitis in childhood. This is a prospective study on 59 children admitted with abdominal pain at St. Mary's Hospital, the Catholic University of Korea from July 2002 to August 2003. We applied Madan Samuel's Pediatric Appendicitis Score (PAS) based on preoperative history, physical examination, laboratory finding and ultrasonography. This study was designed as follows: patients with score 5 or less were observed regardless of the positive ultrasonographic finding, patients with score 6 and 7 were decided according to the ultrasonogram and patients above score 8 were operated in spite of negative ultrasonographic finding. The patients were divided into two groups, appendicitis (group A) and non-appendicitis groups (group B). Group A consisted of 36 cases and Group B, 23 cases. Mean score of group A was 8.75 and group B was 6.13 (p<0.001). Comparing the diagnostic methods in acute appendicitis by surveying sensitivity, specificity, positive predictive value, negative predictive value, and accuracy, PAS gave 1.0000, 0.3043, 0.6923, 1.0000, and 0.7288, and ultrasonography gave 0.7778, 0.9130, 0.9333, 0.7241, and 0.8300 while the combined test gave 1.0000, 0.8696, 0.9231, 1.0000, and 0.9490, respectively. Negative laparotomy rate was 3 %. In conclusion, the combination of PAS and ultrasonography is a more accurate diagnostic tool than either PAS or ultrasonography.