To evaluate the clinical usefulness of abdominal sonography in the diagnosis of large bowel obstruction, the sonographic findings of 39 patients with a large bowel obstruction, in the form of a simple obstruction in 36 patients and a sigmoid volvulus in 3, were reviewed in comparison with their plain X-ray findings. Abdominal sonography showed a large bowel obstruction in 33 patients, and an obstructing lesion in 14 of these patients. However, in the other 6 patients, including the 3 with a sigmoid volvulus, the image was disturbed by extensive colonic gas. Although the plain abdominal X-ray films showed no gaseous colonic dilatation, isolated small bowel dilatation was seen in six patients with a large bowel obstruction proximal to the splenic flexure. In five of these six patients, abdominal sonography revealed a dilated colon filled with fluid and feculent contents which was difficult to evaluate on the plain X-ray films. Consequently, abdominal sonography was proven to be useful, especially for detecting X-ray-negative colonic dilatation.