Seventy-eight young patients with symptoms of chronic inflammatory bowel disease (IBD) of the colon have been investigated to determine to what degree colonoscopy adds important information for the diagnosis of IBD in addition to results of the routine procedures, including rectosigmoidoscopy carried out at the same time. After colonoscopy IBD was established in 4 of 12 patients classified as non-IBD after the routine procedures. Eleven of 18 patients with the routine diagnosis indeterminate colitis (IC) could after colonoscopy be differentiated into UC or probable CC. In 30 of 31 cases classified as UC the routine diagnosis was confirmed by colonoscopy. Routine diagnosis as probable CC was changed to definite CC in 3 of 10 cases. In all cases but one with previously established IBD it was possible to confirm the diagnosis. Thirty-seven of 70 patients with established IBD of the colon had no radiologic evidence of colitis.