Mutations of receptor tyrosine kinases are implicated in the constitutive activation and development of humanhematologic malignancies. Mutations in fms-like tyrosine kinase 3 (FLT3) gene including internal tandemduplication (ITD) and point mutation in the tyrosine kinase domain (TKD) as well as in nucleoplasmin (NPM1)gene are associated with pathogenesis of acute myeloblastic leukemia (AML). Several reports have demonstratedhigh incidences of the FLT3 and NPM1 mutations in adult AML patients. Since the pathogenesis of pediatric AMLis different from that of adult and the FLT3 and NPM1 mutations have not been well characterized in childhoodAML. Therefore, the objective of this study was to determine the frequencies of FLT3 and NPM1 mutations in64 newly diagnosed childhood AML patients. All blood and bone marrow samples were previously diagnosedwith AML by using flow cytometry and/or cytochemistry. FLT3-ITD and FLT3-TKD were detected by PCRand PCR-RFLP methods, respectively. The NPM1 mutation was analyzed by PCR and direct DNA sequencing.The FLT3 mutations were detected in 7 of 64 (11.1%), including FLT3-ITD in 4 of 64 (6.3%) and FLT-TKD in3 of 62 (4.8%). The NPM1 mutation was not detected in this cohort. By multivariate analysis, white blood cellcounts, peripheral blood and bone marrow blast cell counts at diagnosis were significantly higher in childrenwith FLT3-ITD (P<0.05). In addition, the median percentage of CD117 was significantly higher in leukemic blastcells with FLT3-ITD than those with wild type (P=0.01). We did not find any FLT3 mutations in children agedless than 5 years. The AML M3 cell type was most frequently associated with FLT3 gene mutations (50%). Inconclusion, the FLT3 mutations was found in 11.1% but none of NPM1 mutation was detected in Thai childrenwith AML. These data support the hypothesis of different biology and pathogenesis between adult and childhoodAML.