Colonoscopy during an attack of severe ulcerative colitis is a safe procedure and of great value in clinical decision making. Thirty-four patients who had severe ulcerative colitis had a colonoscopic examination during the attack. The indications were resistance to therapy and/or differential diagnostic purposes. Total colonoscopy was performed in 25 cases and less extensive examination in nine. Biopsy was taken in 22 cases. No clinically significant complications occurred. The procedure helped us to postpone immediate surgical intervention in 19 cases. Fourteen of these patients have maintenance treatment and are still well after a follow-up median time of 5 yr (6 months to 10 yr). 