Juvenile myelomonocytic leukemia (JMML) is a myelodysplastic/myeloproliferative disorder of young children. Because the disease is rare and the diagnosis is difficult, a prospective registration of patients suspected of having JMML with a pathological central review have been conducted by the MDS Committee of the Japanese Society of Pediatric Hematology. Between 1999 and 2006, 75 children with JMML were enrolled and diagnosed through this system. Median age at diagnosis was 20 months (1∼85 months). Cytogenetic abnormalities were detected in 21 patients, including 11 with monosomy 7. The 5-year overall survival (OS) was 60%. Regarding the treatment, 61 of the 75 patients received stem cell transplantation (SCT). Conditioning regimen varied widely, and the source of grafts was bone marrow for 43 patients, peripheral blood for 5, and cord blood for 13. The 5-year OS after SCT was 61%. Notably, patients who received cord blood transplantation had inferior survival than those who received grafts from other sources (38 vs. 68%; P=0.03). Given better recognition of the disease, a multi-center protocol study on SCT, JMML11, is now being planned by the Japanese Pediatric Leukemia/Lymphoma Study Group.