Essential thrombocythemia (ET) is rare in children, and little or no information is available about clonality or JAK2 mutations. However, the analyses in this work prove useful for the diagnosis of adult myeloproliferative disorders (MPDs). We evaluated the clonality status and V617FJAK2 mutation in 20 children affected by ET and compared them with 47 consecutive adult ET cases. Clonality was evaluated on the DNA of granulocytes and on the RNA of platelets. V617FJAK2 was analyzed by sequencing tests, allele-specific polymerase chain reaction (PCR), and digestion by BsaXI. A monoclonal pattern was found in 4 (28.5%) of 14 children and in 45% of informative adults. Heterozygous V617FJAK2 was found less frequently in children than in adults (P < .009). Only 2 girls showed both the V617FJAK2 mutation and a monoclonal pattern; one of them was the only child presenting a major thrombotic complication. In contrast to adults, most children with ET do not show either a clonal disorder or the V617FJAK2 mutation.