Juvenile myelomonocytic leukemia (JMML) is a rare clonal myeloproliferative disease of early childhood. To determine the diagnostic features, appropriate treatment, and overall patient survival pertaining to JMML for children, the authors reviewed the clinical data of 16 children with JMML admitted to the National Taiwan University Hospital between 1978 and 2001. Median age at diagnosis was 2.5 years. Fever was the most common symptom at diagnosis. At initial presentation, the mean white blood count and absolute monocyte count were 30 x 10(9)/L and 4.5 x 10(9)/L, respectively. Cytogenetic analysis was performed in 14 patients, and 2 patients (14%) had monosomy 7. Another patient, with normal karyotype at diagnosis, had deletion of 7q22 at the follow-up chromosome study. Forty-seven chronic myeloid leukemia (CML) patients were also diagnosed and followed at the same hospital during the same interval period. The age, leukocyte counts, platelet counts, basophil counts, monocyte percentages on peripheral blood smears, and median survival rate showed significant differences between JMML and CML patients (P < 0.05). The median survival was 10 months and the probability of 10-month survival was 0.38 by Kaplan-Meier analysis for 12 of the 16 JMML patients who did not receive hematopoietic stem cell transplantation (HSCT). Among three patients receiving HSCT, one patient relapsed 9 months after the first HSCT and was treated successfully by a second HSCT from the same sibling donor.