OBJECTIVE. We determined the sonographic features of perforating appendicitis in children in order to determine the best criteria for establishing the diagnosis. MATERIALS AND METHODS. Sonograms of the right lower quadrants of 71 children with proved appendicitis were reviewed to determine the value of sonography in distinguishing between nonperforating and perforating appendicitis. The sonographic signs evaluated included the presence or absence of an appendix, an echogenic submucosal layer, increased periappendiceal echogenicity, free or loculated periappendiceal or pelvic fluid collections, and appendicoliths. The sonographic findings were correlated with the surgical and pathologic findings. RESULTS. Forty-five patients had nonperforating appendicitis, and 26 had perforating appendicitis. A sonographically visible appendix was present in all patients with nonperforating appendicitis and in 10 (38%) of 26 patients with perforation. An echogenic submucosa was noted in 27 (60%) of 45 patients with uncomplicated appendicitis but in only three (30%) of 10 patients with a visible appendix and perforating appendicitis (p < .05). In 19 of 26 patients with perforating appendicitis, sonography showed loculated periappendiceal or pelvic fluid collections; no patient with nonperforating appendicitis had a loculated fluid collection (p < .05). No statistically significant association was found between the presence or absence of perforation and free pelvic fluid, prominent periappendiceal fat, or an appendicolith. CONCLUSION. Our results indicate that sonography can be helpful in the diagnosis of perforating appendicitis. The best predictors of perforation are absence of the echogenic submucosal layer and the presence of a loculated fluid collection.