To determine the sonographic features of uncomplicated acute diverticulitis of the cecum and ascending colon, the sonographic findings in 534 patients who presented with right lower quadrant pain were reviewed. Of these, 18 patients had uncomplicated acute diverticulitis of the cecum and ascending colon. The diagnosis was confirmed by surgery (one patient), clinical course (17 patients), CT (eight patients), or contrast enema (11 patients). On sonography, a round or oval focus of varying echogenicity, which protruded from a segmentally thickened colonic wall and was surrounded by a hyperechoic area, was seen in all 18 patients. These were hypoechoic foci (12 patients), hypoechoic foci with internal strong echoes (three patients), and echogenic shadowing foci with surrounding hypoechoic bands (three patients). Extraluminal gas (one patient) and thickening of lateroconal fascia (six patients) were seen also. Findings of enlarged appendix, frank abscess, and ascites were absent. All patients, including the one who had laparotomy, were successfully treated medically for diverticulitis. Of 515 patients without diverticulitis, in only one patient with acute appendicitis did sonography show a hypoechoic protruding focus. Our experience indicates that the major sonographic finding in patients with uncomplicated acute diverticulitis of the right colon is a hypoechoic round or oval focus protruding from a segmentally thickened colonic wall.