The sensitivity of ultrasonography (US) as the initial imaging procedure in acute diverticulitis of the left colon was retrospectively compared with that of computed tomography (CT) in 34 patients. Final diagnosis was based on clinical findings and follow-up in a first group of 20 patients with mild diverticulitis, and was proved by surgery in a second group of 14 patients with severe diverticulitis. In the first group, US showed findings consistent with diverticulitis in 19 patients, as did CT. The image of an inflamed diverticulum was shown by US in 10 patients. In the surgical group, findings consistent with diverticulitis were shown by US in 11 cases, and by CT in 13 cases. An inflamed diverticulum was not detected at US in this group of patients, presumably because the diverticulum was incorporated in the inflammatory process. These results suggest that US is a valuable technique in the initial evaluation of patients suspected of having acute diverticulitis, especially when the disease is not severe. The image of an inflamed diverticulum shown at US strongly suggests the diagnosis of mild diverticular disease. When diverticulitis is severe, CT should be performed in addition to US.