Two hundred patients with schistosomiasis mansoni, registered in the Infectious and Parasitic Disease Ward, Hospital das Clínicas, Faculdade de Medicina, Universidade de São Paulo, were treated with praziquantel, single oral dose of 40 or 50 mg/kg BWT. Their ages ranged from 6 to 63 years and 33 cases (16.5%) were under 15 years old. The main side-effects were: dizziness 27.5%; drowsiness 21.0%; abdominal cramps 17.5%; nausea 16.0%; diarrhea 9.0%; headache 7.5%; vomiting 4.0%; fever 2.0%; dysentery 1.0%; shivers 1.0%; rash 1.0% and urticaria 0.5%. The drug toxicity was evaluated by laboratory workout — blood counts, hepatic function tests (serum enzymes and bilirubin), kidney function tests (BUN and creatinine), EKG and EEG — performed before as well as after treatment. No relevant abnormality was found leading to clinical manifestations. Success of therapy was assessed in 115 cases through eight consecutive stool examinations performed during the six-month period following the drug administration. Both Hoffman's and Kato-Katz's techniques were used. Rectal biopsy was carried out in 62 patients out of 82 whose stool examinations were negative. Two positive cases were found indicating a 3.2% rate of false-negative results in relation to stool examinations. The cure rates varied from 65.2% in patients under 15 years old to 75.0% in those over this age. The mean rate was 71.3%. These results point out that praziquantel, at the used dose levels, is relatively efficaceous in the treatment of schistosomiasis mansoni and yields a low incidence of side effects and toxic reactions.