Acute appendicitis is a common disorder and, ideally, should be diagnosed prior to the onset of gangrene or perforation. Nonetheless, the goal of early diagnosis remains elusive. In a prospective study, 100 appendectomies were performed for suspected acute appendicitis over 19 months. The location of the appendix was noted by the operating surgeon and was listed as anterior intraperitoneal, retrocecal, pericolic gutter, retroileal, pelvic, or retroperitoneal. The latter four positions were regarded as sites in which the appendix was hidden from the anterior parietal peritoneum. Fifteen patients did not have appendicitis. Of the 85 inflamed appendices, 25 were indurated, 19 were suppurative, and 41 were gangrenous or perforated. Patients with gangrene or perforation were more likely to have pain and tenderness at a site other than the right lower quadrant and had a higher mean heart rate on admission than patients with simple appendicitis, but there were no other differences in symptoms, signs, or laboratory findings among the groups. The appendix was in a hidden location in 15 per cent of patients with simple appendicitis or without appendicitis, compared with 68 per cent of patients with gangrenous or perforative appendicitis (P < 0.001). Complications were more frequent, and hospital stays were longer in patients with advanced appendicitis (P < 0.001). Patients and physicians were equally responsible for delays in treatment, but the high incidence of hidden appendices in those with advanced appendicitis resulted in less severe symptoms and signs than expected. Anatomic variations in the location of the appendix are often responsible for delays in the diagnosis of appendicitis.