Forty-one patients admitted to our hospital during an 18-month period with the clinical diagnosis of colonic diverticulitis were analyzed to evaluate the utility of computed tomography (CT). Abdominal pain and leukocytosis were the most common presenting manifestations, 75 and 66%, respectively. Just over one-half of the patients also demonstrated hematuria. Twenty patients required surgical intervention, most commonly for failure to improve despite medical management. Preoperative studies included 10 sigmoidoscopies, 30 plain abdominal roentgenograms, 20 barium enemas (BE), and 24 CT scans. Two CT scans were also obtained after operation for successful percutaneous drainage of intra-abdominal abscesses. Abdominal roentgenograms were most commonly obtained but least helpful, with only one third demonstrating any abnormality whatsoever. Sigmoidoscopy was least commonly performed but almost universally abnormal. Specificity was low, however, in that spasm with inability to advance the endoscope was the most common finding. Of the 20 barium enemas obtained, 60% had findings consistent with diverticulitis, most commonly localized perforation or fixed narrow segment. Sixty-three per cent of CT scans were abnormal. The most frequent findings were localized thickening of the colonic wall and increased density in the pericolic fat. Diverticular abscess, which may be inferred by other studies, was definitely diagnosed in one third of the patients with abnormal CT scans. CT also provided the ability to identify extracolonic intra-abdominal pathology. The study demonstrates that both barium enema and CT are effective in diagnosing diverticulitis, although CT can be performed without risk. CT played no therapeutic role before operation, although two patients benefited after operation by CT-guided drainage of intra-abdominal abscesses. The decision for surgery was most frequently dependent on clinical examination and never solely on the basis of either the barium enema or CT in this study. The major benefit of CT appears to be its ability to identify both gross and subtle changes indicative of diverticular disease and extracolonic pathology in a relatively noninvasive manner.