['To determine the prevalence of perforated and nonperforated appendicitis in patients with nonvisualization of the appendix on ultrasound (US) performed for suspected appendicitis, and to evaluate the value of CT in these patients.', 'We analyzed 400 consecutive patients undergoing US for suspected appendicitis. Of these patients, 260 had nonvisualization of the appendix, but otherwise normal scans. We analyzed the clinical outcome in these patients to determine the prevalence of appendicitis, referrals for CT, and the contribution of CT in these patients.', 'Of the 400 patients, 140 (35%) had either a normal (80 patients, 25%) or an abnormal appendix (60 patients, 15%); 260 (65%) had nonvisualization of the appendix. Overall 75 patients had appendicitis (18.8%) and 17 (4.3%) had appendicitis with perforation. Of the 260 patients with nonvisualization of the appendix, 14 patients (5.4%) had appendicitis and 2 were perforated (0.8%). The prevalence of perforated and nonperforated appendicitis in this group was significantly lower than the overall group (p < 0.001 and p < 0.01, respectively). Of these 260 patients, 101 patients (38.8%) had CT within 48 hours and 79 (78.2%) had normal scans.'] Is sonography for appendicitis : nonvisualization of the appendix an indication for active clinical observation rather than direct referral for computed tomography?