Background. Schistosome infections are often clinically silent, but some individuals develop severe pathological reactions. In several disease processes Th17 cells have been linked to tissue injuries, while regulatory T (Treg) cells are thought to down-modulate inflammatory reactions. We assessed whether bladder pathology in human Schistosoma haematobium infection is related to the balance of Th17 and Treg cells. Using a murine model of Schistosoma mansoni infection we further investigated whether the peripheral profiles reflected ongoing events in tissues. Methods. We characterized T helper subsets in the peripheral blood of children residing in a S. haematobium-endemic area and in peripheral blood as well as in spleen and hepatic granulomas of S. mansoni-infected high-pathology CBA and low-pathology C57BL/6 mice. S. haematobium-infected children with bladder pathology had a significantly higher percentage of Th17 cells than those without pathology. Moreover, the Th17/Treg ratios were significantly higher in children with pathology compared to infected children without pathology. Results. Percentages of IL-17-producing cells were significantly more abundant in the spleen and granulomas of CBA compared to C57BL/6 mice. This difference was also reflected in the peripheral blood. Conclusions. Our results indicate for the first time that Th17 cells may be involved in the pathogenesis of human schistosomiasis.