Zsófia Váradi,1 Rita Bánusz,1 Judit Csomor,2 Krisztián Kállay,3 Edit Varga,1 Gabriella Kertész,1 Monika Csóka1 1Second Department of Pediatrics, 2First Department of Pathology and Experimental Cancer Research, Semmelweis University, 3Pediatric Hematology and Stem Cell Transplantation Unit, United St István and St László Hospital, Budapest, Hungary Abstract: Erdheim–Chester disease (ECD) is a rare histiocytic disorder, characterized by the xanthomatous infiltration of tissues by CD68-positive and CD1a-/CD100-negative foamy histiocytes. In childhood, ECD is exceptionally rare, and only a dozen cases have been published so far. The cooccurence of Langerhans cell histiocytosis (LCH) and ECD is even rarer. Here, we report a 2-year-old boy, the youngest patient in the literature so far, who was diagnosed with concomitant BRAF mutation-positive LCH and ECD. In his case, conventional LCH treatment proved to be ineffective, but he is the youngest patient who was successfully treated with the BRAF inhibitor vemurafenib. Keywords: vemurafenib, treatment, Erdheim–Chester disease, Langerhans cell histiocytosis, pediatric histiocytic disorders