The JAK2 V617F mutation is present in most patients with polycythemia vera, but in fewer patients with essential thrombocythemia (ET). We have assessed the frequency of this mutation in ET patients using amplification refractory mutation system PCR and determined the relationship of the mutation with disease phenotypes. Clinical-laboratory findings and histomorphological features were compared according to mutational status in 108 ET patients. The mutation was detected in 61 patients (56.5%) including one homozygous patient. Those with the mutation had significantly higher leukocyte (P = 0.003) and neutrophil (P = 0.007) counts. However, the incidences of thrombotic events and progression to advanced stages did not differ significantly between patients with and without the mutation. Thrombotic events were significantly correlated with older age (P = 0.025). Morphological analysis revealed that erythroid hypoplasia was exclusively found in the mutation-negative patients (P = 0.027). We could confirm previous findings of higher leukocyte count in ET patients with JAK2 mutation, but could not find any correlation with thrombotic events. Therefore, the detection of the mutation could characterize a subset of ET patients with distinct phenotype, despite its clinical significance being still undetermined.