In a prospective study 404 unselected patients suspected of having acute appendicitis were examined by ultrasound, the results being compared with the initial clinical findings. Acute appendicitis was established in 27.2%, confirmed by histology. The overall accuracy of ultrasonography in relation to the diagnosis of appendicitis was 95.5%, 95.1% when positive, 95.7% when negative. An acutely inflamed appendix could not be demonstrated by ultrasound in 13 patients (sensitivity 88.2%), while in five there was a false-positive ultrasound result (specificity 98.3%). Complicated appendicitis with perforation occurred in 22 patients (20%): ultrasound sensitivity in this situation was 91%. Especially when the clinical picture was unclear (34.5% of all cases of appendicitis) a definitive diagnosis of acute appendicitis was made both earlier and more certainly by ultrasonography. Further-more, in many instances ultrasound was able to demonstrate other serious disease in the differential diagnosis from acute appendicitis. Negative laparotomy rate was lowered from 21.9% to 11.4% by taking into account ultrasonographic findings. Thus ultrasonography proved to be an important method as an imaging technique and in the differential diagnosis of acute appendicitis.