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The overlap of Erdheim–Chester disease (ECD) and Langerhans cell histiocytosis (LCH) is more common than it was generally accepted. Both diseases seem to be linked by a mutation in oncogenic BRAFV600E, probably an early event which occurs in bone marrow progenitor cells. In this article are described the clinical and histological findings in 2 cases of ECD-LCH overlap syndrome bearing the BRAFV600E mutation in both ECD and LCH lesions in bone and skin. In one case, lesions of ECD and LCH were situated directly site-to-site in the same bone section leading to the assumption of a common myeloid precursor cell for these diseases. Furthermore, we focus on the histopathological diagnostic criteria of cutaneous involvement in ECD. Lesional tissue shows a dermal infiltrate of lipidized CD68+, CD163+, CD1a−, and langerin histiocytes admixed with Touton giant cells—a xanthogranulomatous phenotype. Often, this pattern of histopathology requires correlation with patterns of systemic involvement to differentiate ECD from other xanthogranulomatous infiltrates. This endeavor is of major importance to determine early diagnosis and treatment, because ECD often shows a poor prognosis compared with its differential diagnoses. Finally, adults who suffer from LCH and develop xanthogranulomatous infiltrates should always be screened for ECD-LCH overlap syndrome.