The diagnostic accuracy of high-resolution real-time ultrasonography was prospectively studied in 240 patients admitted to the hospital with suspected acute appendicitis. The criteria for ultrasound diagnosis of appendicitis included a sausage-shaped, aperistaltic, hypoechoic structure when imaged along its longitudinal axis, with a target-like appearance on transverse section. The ultrasonographic findings were correlated with surgical-pathological outcome in 82 cases with proven appendicitis, with laparotomy findings in another 21 patients and with clinical follow-up in the remainder. The overall sensitivity, specificity and accuracy of ultrasonography in the diagnosis of acute appendicitis were 78%, 92% and 87%, respectively. The positive predictive value was 84% and the negative predictive value was 88%. Ultrasonography may significantly improve the diagnostic accuracy in patients with suspected acute appendicitis and should be performed in all patients in whom the clinical diagnosis is equivocal. The ultrasonographic findings must be interpreted in light of the clinical findings.