Th17 cells and their effector cytokines have emerged as important mediators in inflammatory and autoimmune diseases and serve as an ambitious field in current immunology research. Recent studies suggest a potential impact of Th17 cells on solid tumors but relatively little is known about their contribution in hematological malignancies. The current study was designed to investigate the possible involvement and clinical significance of circulating Th17 cells in acute leukemia. Flow cytometry was used to analyze percentages of Th17 cells in peripheral blood mononuclear cells from 93 acute leukemia patients (ALL, n = 30; AML, n = 63) and 40 healthy volunteers. Serum levels of IL-17 and IL-21 were measured using enzyme-linked immunosorbent assay. Circulating Th17 cells were increased in patients with acute leukemia (2.88 ± 0.65% and 2.90 ± 0.57% in ALL and AML patients, respectively) and were significantly higher than in healthy controls (1.10 ± 0.28%; P = 0.001). Furthermore, pretreatment Th17 cells were reduced significantly in patients who achieved complete remission after induction therapy (2.25 ± 0.44 % and 1.63 ± 0.27% in ALL and AML patients, respectively, P < 0.0001). Serum levels of IL-17 and IL-21 were significantly elevated in acute leukemia patients. Kaplan-Meier curves revealed a significantly longer overall survival in patients with high Th17 levels (P = 0.029 and P = 0.027 for ALL and AML, respectively). In the multivariate analysis, Th17 cells retained statistical significance for overall survival in patients with ALL (OR 0.331; P = 0.043) and AML (OR 0.489; P = 0.032). These results strongly suggest Th17 cells as a powerful new prognostic determinant which could serve as a potential therapeutic target to modulate anti-tumor response in acute leukemia patients.