Ninety-four patients, 22 males and 72 females, from 11 to 71 years old, with an average of 25 years, presenting the hepatosplenic form of Schistosomiasis mansoni were treated with a new anthelmintic drug — praziquantel — to investigate its efficacy and tolerance. In addition to the hepatosplenic form of the disease, 14 patients had the cardio-pulmonary hypertensive or cyanotic form and 16 had already been submitted to splenectomy. Two oral doses — 1 x 30 and 2 x 25 mg/kg body weight — were compared. Side-effects were noticed during the first two days following the drug administration but usually of mild intensity and short duration. The most frequent and, at times, more severe were abdominal pain or discomfort, diarrhea, headache, dizziness and nausea. Fever was present in 19.2% of the cases and urticaria and pruritus in two patients. The laboratorial investigation of liver function showed, in some cases, slight enzymatic alterations (AST, ALT, y GT) 24 hours after medication, but without any major significance. No urinary, blood glucose or hematological changes have been detected, except, for the commonly seen increase of eosinophyls on the 7th and 30th day, related to the death of parasites inside the organism. Likewise, no abnormality was noticed in the electroencephalografic study. In the electrocardiogram it was observed in two patients a slight and transitory modification on the ventricular repolarization. With regard to the parasitological cure, it was observed in 62 patients Who have finished the 6 months control period, a total cure rate of 80.6%. With 30 mg/kg single dose this cure rate was 76.7% and with 25 mg/kg b.i.d. 84.4%. The non-cured patients, on the other hand, have shown a marked reduction in the number of S. mansoni eggs eliminated in the feces. Furthermore, five of these patients Were retreated six or more months afterwards with the same initial dose, achieving 100% negative stool examinations. The Authors believe that administering a higher dose, like 60 mg/kg, it would be possible to accomplish, in addition to good tolerance, a greater percentage of parasitological cure. The long-term clinical and laboratory improvement, observed 6 and 12 months after treatment, is worthy of note.