Hyperbaric oxygen therapy has been used to successfully treat perineal Crohn's disease. We describe the first successful use of hyperbaric oxygen therapy in the treatment of ulcerative colitis, refractory to conventional therapies. Therapy consisted of 30 courses of 100% oxygen at a pressure of 2.0 atm absolute. Clinical remission was achieved on the basis of the Truelove-Witts and disease activity index scores. Corticosteroids were successfully tapered off once remission was achieved.