The diagnostic accuracy and practical impact of high-resolution sonography were prospectively studied in 523 consecutive patients admitted to the hospital with suspected appendicitis. The criteria for ultrasound diagnosis of acute appendicitis included visualization of a noncompressible aperistaltic appendix, with a targetlike appearance in transverse view and a diameter greater than or equal to 7 mm. In 115 of 130 patients with proven appendicitis the inflamed appendix or appendiceal abscess could be visualized, giving a sensitivity of 88.5%. The mean diameter of ultrasonically visible appendices was 11.4 +/- 3.2 mm. The overall accuracy and specificity of sonography in the diagnosis of acute appendicitis were 95.7% and 98%, respectively. The predictive value of a positive test was 94.5% and that of a negative result 96.3%. In a separate analysis of the results in 121 women of childbearing age, who have a high risk of preoperative misdiagnosis, the overall accuracy was found to be 96.7%, with 82.6% sensitivity and 100% specificity. Twenty-four (89%) of the 27 patients with appendiceal rupture (incidence 20.8%) were correctly diagnosed with ultrasound. The other 3 cases (11%) were missed. Routine use of ultrasonography has significantly improved the diagnostic accuracy in patients with suspected appendicitis and has reduced the negative laparotomy rate from 22.9% to 13.2%.