We described a two-year-old boy who developed a skin infiltration from JMML. Several indurated erythematous lesions were seen on his back on his first visit to our department. Edematous erythemas had repeatedly appeared on his auricles and feet for the previous six months. He had had a high fever for a month. Hepatosplenomegaly and superficial lymphadenopathy were recognized. Laboratory investigation showed leukocytosis and anemia. The diagnosis of JMML was confirmed by the findings of myeloid hyperplasia in his bone marrow and the spontaneous colony formation and GM-CSF hypersensitivity in a culture of bone marrow cells. Histopathologically, large atypical mononuclear cells were infiltrated throughout the dermis in a perivascular and interstitial distribution in a skin biopsy specimen. These cells were CD3 (-), CD20 (-), CD45 (+), CD68 (+) and myeloperoxidase (+). Bone marrow transplantation and then cord blood stem cell transplantation were performed but soon rejected. The indurated erythematous lesions appeared again soon after the relapse of JMML. There are other reported cases of JMML with skin infiltration that preceded any other manifestations of the disease. JMML cells in some patients, including our case, seem to have a great affinity for the skin, and skin biopsy aids in early detection of this disease.