Juvenile myelomonocytic leukemia (JMML) is a mixed myeloproliferative and myelodysplastic disorder that occurs in early childhood. The clinical course of JMML is highly variable. A third of patients follow a relatively indolent course, although approximately 15% cases are thought to develop acute myeloid leukemia, referred to as blast crisis. The etiology and clinical characteristics of blast crisis remain unclear. We document the case of a 27-month-old boy who presented with hepatosplenomegaly, skin rash, and lymphadenopathy. An initial diagnosis of acute erythroid leukemia was made according to the French-American-British classification. Following estimation of hypersensitivity to GM-CSF and genetic analysis of PTPN11, he was diagnosed with JMML in the blast crisis phase. Although he had several poor prognostic factors, including monosomy 7 and high HbF percentage, he achieved partial remission after treatment with acute myeloid leukemia-oriented chemotherapy followed by allogeneic hematopoietic stem cell transplantation. He has been in complete remission for over 6 years.