A patient with severe diarrhoea and gas-filled cysts in the wall of the colon is reported. The patient had normal lung function but an extremely high end-expiratory H2 fasting value. Treatment with an elemental diet for 2 weeks resulted in a decreased H2 value and in regression of the cysts. After 4 months on a normal diet extensive recurrence was observed. We conclude that excessive intestinal gas production may play a role in the pathogenesis of the cysts.