Registry for Patients With Erdheim-Chester Disease and Other Histiocytoses

Inclusion Criteria: * Diagnosis of Erdheim-Chester Disease, Langerhans cell histiocytosis, Rosai Dorfman disease, or another histiocytic neoplasm by either 1) documented histopathologic establishing ECD or HN or 2) documented histopathologic findings compatible with Erdheim-Chester disease or HN in the context of corroborating clinical and/or radiologic findings or 3) documented radiologic findings, in the absence of a biopsy having been performed or yielding evaluable tissue, that are felt by the Principal Investigator to unequivocally represent an ECD/HN diagnosis given the clinical context or 4) self reported with supporting documentation upon medical record collection. * Proficiency in English, in the determination of the Investigator or by self report. * Willing to have historical and future HN-related health records sent to Registry review. Exclusion Criteria: * Patients unwilling to sign consent. * Participants under the age of 18.