To date, the diagnosis of acute appendicitis (AA) has been made primarily on clinical grounds and imaging techniques have been considered less useful. This paper is aimed at evaluating US utility in the diagnosis of AA. US scans of the appendix, after Puylaert, were obtained in 30 healthy subjects (group A), in 49 patients with recurrent pain in the iliac fossa (group B), and in 28 patients with clinical signs highly suggestive of AA (group C). In none of the subjects in group A was the appendix seen. Neither was it demonstrated in the patients in group B, all of whom underwent surgery and never presented with suppurating, phlegmonous, gangrenous, perforating AA and/or abscess. In the patients in group C, 20 of whom had a positive histology for AA, US demonstrated 17 true positive, 8 true negatives and 3 false negatives. We consider as useful in the diagnosis of AA the following US signs: 1) the appendix cannot be demonstrated, in which case AA is to be excluded; 2) the appendix can be seen and is still, non-compressible and exhibits thickened walls; 3) the appendix contains fecaliths. The latter 2 findings are suggestive of AA. Thus, US of the appendix is useful (100% specificity in group B) in excluding AA when clinical diagnosis is questionable, but the method proves less reliable (85% sensitivity in group C) when clinical presentation is more definite.