Complications that might lead to surgery in severe attacks of ulcerative colitis have been found to be correlated with the depth of colonic ulcerations as measured by pathological examination of colectomy specimens. In order to evaluate the value of colonoscopy for the assessment of colonic ulcerations, we have reviewed the clinical, biological, colonoscopic, and anatomical findings in 85 consecutive patients with attacks of ulcerative colitis involving at least the rectosigmoid and part of the descending colon, seen in our center between 1981 and 1989. All had colonoscopy performed by a senior endoscopist at entry. Extensive deep colonic ulcerations were diagnosed in 46 of them, and moderate endoscopic colitis in 39. No complication related to colonoscopy occurred except for one colonic dilatation. Forty-three of the 46 patients with severe endoscopic colitis were operated upon; 38 of them failed to improve with high-dose corticosteroids and five had a toxic megacolon. Extensive ulcerations reaching at least the circular muscle layer were found at pathological examination of colectomy specimen in 42 of the 43 patients. Conversely, 30 of 39 patients with moderate endoscopic colitis went into clinical remission with medical treatment, and only nine patients needed further surgery because of medical treatment failure. Six of these nine patients underwent another colonoscopy prior to colectomy, and all six showed features of severe endoscopic colitis. Deep ulcerations reaching the circular muscle layer were found at pathological examination in five of these six patients and in one additional patient whose colonoscopy had been performed 21 days before colectomy.(ABSTRACT TRUNCATED AT 250 WORDS)