 The primary objective of this project is to evaluate the efficacy and safety of praziquantel 60 mgkg in the treatment of schistosomiasis, as compared to the standard 40 mgkg therapy in a representative community from a highly endemic area of schistosomiasis in Northeastern Brazil. Cure rates, reduction in egg counts and proportions of reported side-effects in children at the 10-19 y age-range with at least 100 eggs per gram of faeces will be compared between regimens, aiming to evaluate the superiority of 60 mgkg over the 40mgkg dose currently recommended by the WHO. Reinfection rates will also be evaluated aiming to improve transmission control within the local health system, including re-treatment combined with auxiliary control measures. Features related to the clinical, nutritional and immunological status of the patients prior to treatment will also be investigated in association with the outcome of praziquantel treatment. 