Erdheim-Chester disease (ECD) is a rare non-Langerhan's cell histiocytosis disorder characterized by replacement of normal tissue by lipid-laden histiocytes affecting various organs. A few pediatric cases have been reported worldwide. Here we present a child with leukemia who was diagnosed as ECD. A 2-year and 11-month old boy diagnosed with high risk acute lymphoblastic leukemia (ALL) at the age of 17 months, received allogeneic hematopoietic stem cell transplantation (HSCT) at the age of 2 years old. Six months after the transplantation, the patient was admitted to the hospital with palpable left calf nodules. Bone marrow study suggested ECD without leukemia with complete chimerism status. Excisional biopsy of the left calf nodule showed ‘aggregation of non-Langerhan's cell type epitheloid histiocytes’; clinically suggestive of ECD. The patient was started on vinblastine and corticosteroid treatment.