 The rare histiocytic disorders RHDs are characterized by the infiltration of one or more organs by non-LCH histiocytes. They can range from localized disease that resolves spontaneously, to progressive disseminated forms that can be sometimes life-threatening. Since they are extremely rare, there is limited understanding of their causes and best treatment options. Physicians, patients and parents of children with RHDs frequently consult members of the Histiocyte Society regarding the best management of these disorders. Very often, no specific recommendation can be made due to the lack of prospective outcome data, or even large retrospective case series. The creation of an international rare histiocytic disorders registry IRHDR could facilitate a uniform diagnosis of the RHDs, as well as the collection and analysis of the clinical, epidemiological, treatment and survival data of patients with RHD. The registry may also lead to future therapeutic recommendations, provide a framework for future clinical trials and create excellent research opportunities. 