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T helper type 17 (Th17) cells, which represent a novel subset of CD4+ T cells, play an active role in inflammatory and autoimmune diseases. Recent studies have also suggested that they have an impact on solid tumours. However, the nature of Th17 cells in haematological malignancies remains unknown. In this study, we investigated Th17 cell frequency and secretion of related cytokines in patients with acute myeloid leukaemia (AML). First, we found that Th17 cell frequencies were increased significantly in peripheral blood samples from untreated patients with AML, compared with those from healthy volunteers. Moreover, increased interleukin (IL)-17 concentrations accompanied the increased Th17 cell frequencies in these patients. These results suggest that Th17 cells may play a role in the pathogenesis of AML. Secondly, we found that the increased Th17 cell frequencies were reduced when patients achieved complete remission after chemotherapy, suggesting that measurement of Th17 cell frequencies may have clinical value in the evaluation of therapeutic effect. In addition, we found that IL-6 and transforming growth factor (TGF)-β1 concentrations increased in the untreated patients and that IL-6 concentrations showed a positive correlation with the frequencies of Th17 cells, suggesting that IL-6 may play an important role in Th17 cell differentiation in patients with AML.