A 57-year-old man became aware of left supraclavicular lymph node swelling, which was subsequently diagnosed as Langerhans cell sarcoma, based on a positive immunophenotype for CD1a, S-100 protein, and langerin, and histologically bizarre pleomorphism. The tumor became leukemic 3 months later. Despite intensive chemotherapy, he died of disease progression 7 months after the initial diagnosis. Tumor cells in the leukemic phase expressed CD5, CD7, CD13, CD33, CD34, CD68, and CD123. These findings suggested leukemic transformation from Langerhans cell sarcoma. Leukemic transformation may be a clinical manifestation of advanced Langerhans cell sarcoma, and should be differentiated from acute myelogenous leukemia.