To determine the frequency of visualization of the arrowhead sign and an inflamed diverticulum on abdominal computed tomographic (CT) scans in patients with acute colonic diverticulitis and to assess the specificity of these signs. One hundred fifty consecutive patients suspected of having diverticulitis prospectively underwent helical CT after the administration of contrast material by means of an enema. Sixty-four patients (43%) had a final clinical diagnosis of colonic diverticulitis. Each scan was reviewed for the arrowhead sign, an arrowhead-shaped collection of contrast material within focal colonic wall thickening, and for an inflamed diverticulum, a rounded, paracolic outpouching centered within fat stranding. The arrowhead sign was noted at CT in 17 (27%) of the 64 patients with diverticulitis. An inflamed diverticulum was noted in 21 patients (33%) at CT, with either calcium or similar high-attenuation material in 10 (48%) of the 21 patients, soft-tissue attenuation in six patients (29%), air attenuation in four patients (19%), and barium attenuation in one patient (5%). Neither sign was demonstrated in patients with alternative conditions (100% specificity). The arrowhead sign and an inflamed diverticulum are occasional, specific CT signs of colonic diverticulitis. Their identification can add diagnostic specificity to CT of acute colonic processes.