The decision to operate was made by the surgeon on duty on the basis of clinical suspicion, after abdominal CT or ultrasound. No scoring systems were used during the data collection period. The optional diagnostic imaging performed as well as the surgical method for appendectomy (open or laparoscopic) was at surgeon’s discretion. The diagnosis of acute appendicitis was based on histological examination showing transmural infiltration of neutrophils in the appendix. At the study hospital, all macroscopically normal appendices are invariably removed for histological examination if no other significant pathology is found during the operation. The operative procedures were therefore in all cases, where appendicitis was preoperatively suspected, registered as appendectomies - not as diagnostic laparoscopies. For the study, complicated appendicitis was defined as perforated appendix or appendicular abscess. Three patients with appendicular abscess were initially treated non-operatively and thus did not have histopathological diagnosis. They were classified as acute appendicitis based on CT findings.