The relationship between clinical disease activity, rectal inflammatory activity and mucosal inflammation established at total colonoscopy has been studied in 36 young patients with ulcerative colitis (UC) and 24 with Crohn's colitis (CC). Semiformed stools, diarrhea and frequent bowel movements seem to be better indicators of extensive mucosal inflammation of a moderate or severe degree than blood in faeces. There were, however, several patients with a discrepancy between the degree of clinical disease activity and the extent and degree of mucosal inflammation at endoscopic and histologic assessment. In about one-third of the patients with UC and in about half of the patients with CC, who had mucosal inflammatory activity of a moderate or severe degree, the rectal inflammatory activity was of a lower degree. Total colonoscopy seems to be necessary in order to establish the extent and degree of mucosal inflammation.