Encapsulating peritonitis or "abdominal cocoon" is characterised by part or all of the digestive tube being encased in a thick membrane forming a sort of "cocoon". In addition to secondary forms related to peritoneal dialysis or those with drug related, inflammatory or tumour aetiologies, rare idiopathic forms have been described. We report here a case of an idiopathic form in a 15-year-old adolescent girl.