ABSTRACT. Gain of function mutation of Janus kinase 2 (JAK2V617F) has been identified in Philadelphia-negative myeloproliferative diseases; about half of essential thrombocythemia (ET) patients harbor this mutation. The activated JAK-STAT pathway promotes cell proliferation, differentiation and anti-apoptosis. We studied the role of negative regulators of the JAK-STAT pathway, PIAS, and SOCS in ET patients. Twenty ET patients and 20 healthy individuals were enrolled in the study. Thirteen of the ET patients harbored the JAK2V617F mutation based on mutation analysis. Quantitative-PCR was applied to assay the expression of SOCS1, SOCS3, PIAS1, PIAS3. The expression levels of PIAS1 and PIAS3 were significantly lower in ET groups than that in normal individuals. There was no significant difference between JAK2V617F (+) and JAK2V617F (-) patients. SOCS1 and SOCS3 expression did not differ between ET patients and normal individuals, or between JAK2V617F (+) and JAK2V617F (-) patients. We suggest that failed negative regulators of the JAK-STAT pathway take part in the pathomechanism of ET.