In this study, our results provided direct evidence that PV patients presented a significant increase of circulating Th17 cell percentage, Th17-specific transcription factor RORγt mRNA expression, and its effective cytokine IL-17 mRNA expression and serum levels, while a dramatically decreased proportion of Treg cells and expression levels of Treg-specific transcription factor Foxp3 and its effective cytokine IL-10, which results in an imbalanced ratio of Th17/Treg and RORγt/Foxp3, then confirms that Th17/Treg immune imbalance exists in PV patients. The gene discussed is FOXP3; the disease is acquired polycythemia vera.