Two patients with pneumatosis cystoides coli associated with chronic pulmonary disease are described. In both patients, cysts and symptoms recurred after successful treatment with O2. Both patients had exceptionally high expiratory H2 fasting values. On the basis of these and other observations a new hypothesis is proposed for the pathogenesis of the cysts and for their frequent association with chronic pulmonary disease. It is suggested that the cysts result from excessive colonic gas production in a patient unable to excrete this excess gas via the lungs.