Oltipraz was administered orally to 62 hospitalized male Sudanese infected with Schistosoma mansoni. The patients were split into two equal groups; one group received a total dose of 25 mg/kg body weight, the other group received 35 mg/kg. Half of the total dose was given with breakfast, the second half with supper. In general, the drug was well tolerated although some vomiting was observed 3--5 hours after the second half-dose. Blood chemistry and hematology remained normal 24 hours after administration of oltipraz. The cure rate was above 95% for both groups at 1, 3, and 6 months after treatment. Stratification of patients by eggs/g feces clearly indicated that the drug was equally efficacious for patients excreting high, medium or low numbers of eggs. Our results indicate that further trials will be necessary with lower doses of oltipraz in order to determine its antischistosomal potency.