Langerhans cell histiocytosis (LCH) is a rare disease of the myeloid precursor cells, it predominantly occurs in the skull and long bones as unifocal bone lesions. Aneurysmal bone cysts (ABC) are benign, expansive and lytic bone. Reports of secondary ABC occurring in LCH are rare, having only been reported twice in the skull. Here, we report the first case of LCH masquerading as ABC in a 14-month-old female child who presented with a rapidly growing mass in her left femur. The lesion had typical radiological features of ABC, and only histological examination revealed the presence of cells suggestive of LCH.