From 1989 to 1995 high-resolution ultrasonography (US) was performed in 3,546 children (age: 1-17 years) with clinically suspected appendicitis. A total of 518 patients underwent laparotomy; 420 had histologically proven acute or perforated appendicitis (prevalence 11.8%). In these children, the sensitivity, specificity and overall accuracy of US examination were 90%, 97% and 96% respectively. The use of US in clinically doubtful acute abdomen may allow earlier diagnosis of acute appendicitis; in 1995 the rate of unnecessary appendectomy was reduced to 13%.