Langerhans cell histiocytosis is classically considered as totally different from Chester-Erdheim's disease which consists in the infiltration of various parenchymas by macrophagic CD68-positive histiocytes. We report the case of a 46-year-old woman with a long history of diabetes insipidus who presented typical lesions of Langerhans cell histiocytosis on vulvar and skin biopsies as well as bony cellular infiltrates characteristic of Chester-Erdheim's disease. A few months later she presented cerebellar disorders and died after an 18-month course. At autopsy the pons was enlarged, due to numerous cellular infiltrates which were also scattered in the middle cerebellar pedoncles, dentate nuclei, midbrain and hypothalamus. There were S100-protein positive Langerhans cells intermingled with numerous ovoid CD68-positive histiocytes. There are a few reported cases of Chester-Erdheim's disease presenting foci of Langerhans cells histiocytosis in other parenchymas. In addition, there are 10 reported cases with diabetes insipidus and bilateral infiltration of the brain stem and cerebellum, considered as presenting either one type of histiocytosis or the other. Our case demonstrates that both histiocytoses may coexist in the brain and thus correspond in fact to the same pathology in certain particular cases.