A 9‐year‐old girl with leukemic infiltration of cells characterized by indented nuclei and abundant faintly acidophilic cytoplasm was autopsied. Electron microscopic studies of the cervical lymph nodes and tumor nodules in the liver and kidney (fixed in formalin solution for 3 years) together with the specimens taken from the cutaneous eruptions at autopsy demonstrated that the proliferating cells frequently contained Langerhans' granules in the cytoplasm. Accordingly, this case was thought to be a disease with systemic, progressive, and malignant proliferation of Langerhans' granule‐containing histiocytes (LGH). The intimate relationship between Letterer‐Siwe's disease and LGH has recently been recognized. Inasmuch as the present case satisfied both the clinicopathologic criteria of Letterer‐Siwe's disease and malignant histiocytosis, the proliferating histiocytes in both of them could be the same type of cell (LGH).