PURPOSE: Aim of the study was to describe changes in the epidemiology of acute appendicitis in Danish children between 0-19 years of age for the period 1996-2004. METHODS: The study was based on discharge diagnoses taken from the Danish National Patient Registry of all 28 274 patients with a diagnosis of acute uncomplicated or complicated appendicitis, and/or a registered procedure code of appendectomy. These data were computed together with data on the background population, and incidences were calculated. RESULTS: A significant decrease in the incidence of acute uncomplicated appendicitis was found for all age groups (range, 13-36%). The decrease was present for both sexes, but most prominent in girls. The incidence of complicated acute appendicitis decreased by 10%. CONCLUSION: The incidence of acute appendicitis is declining. The incidence of uncomplicated appendicitis appears to be declining more rapidly than that of complicated appendicitis. This finding does not necessarily imply poorer management of the disease, but-as suggested by others-could be due to different behaviours of uncomplicated and complicated appendicitis.