Purpose: To report the unusual occurrence of essential thrombocythemia
(ET) in a 5-month-old infant. Patients and Methods: The child was ref
erred by her pediatrician for a high platelet count detected on routin
e blood testing. The child was asymptomatic except for failure to thri
ve. Diagnostic tests to rule out secondary causes of thrombocytosis as
well as myeloproliferative syndrome were done. Results: The platelet
count persisted at >1,000,000/mu l, and no secondary cause could be es
tablished. The bone marrow aspirate and biopsy showed normal cellulari
ty, no fibrosis, orderly maturation, and normal morphology of all thre
e cell lines, with only an increase in the number of megakaryocytes. A
diagnosis of ET was established as per the Polycythemia Vera Study Gr
oup criteria. In view of the young age of the patient and asymptomatic
course of the disease, we decided to observe the child only, reservin
g antiplatelet agents for use in the event of a thromboembolic or hemo
rrhagic episode. Conclusions: This is the youngest known child with ET
. We believe that conservative observation is warranted in an asymptom
atic child. The child is currently doing well and is asymptomatic 4.5
years after diagnosis.