DISCUSSION: Histiocytic neoplasms are myeloproliferative disorders characterized by MAPK pathway alterations. A subgroup are termed LCH while the remainder are termed non-LCH disorders (ECD included). LCH and ECD are clinically similar, with a preference for skin and bone, but otherwise heterogeneous organ involvement, with lung lesions more common in ECD (Ref 1). On biopsy LCH and ECD both have dendritic cells, histiocytes and multinucleated giant cells. ECD cells express histiocyte markers and Factor XIIIa but, unlike LCH, only scant S100 and no CD1a. Treatment for LCH and ECD includes steroids and Interferon, respectively. Prognosis of overlap histiocytosis is worse than ECD (which is worse than LCH). While MDS often occurs with Histiocytic neoplasms (possibly due to common mutations or clonal origin), to our knowledge this is the first published case of Overlap histiocytosis causing ILD in an LCH patient with MDS.