The case reported is that of a young woman who underwent a termino-terminal jejno-ileal by-pass procedure for obesity which was refactory to usual forms of treatment. Eight months later, a cholecystectomy was carried out for lithiasis, presenting with abdominal pain. At the time of operation, lesions of cystic pneumatosis were discovered on the excluded length of small bowel. This complication frequently manifests itself in the form of pseudosurgical abdominal pain, or as diarrhoea. More rarly, it is a radiological finding. The pathogenesis remains a subject of discussion. The mechanical theory would seem the most logical, since colonic intraluminal pressure is higher than that in the intestine excluded from the circuit. Bacterial proliferation, classical in blind loops, would be a farourising factor.