Twenty male patients (mean age 23 years) with Schistosoma mansoni infections (mean egg count 429 ± 311/g feces) were each treated with oxamniquine orally in a single daily dose of 20 mg/kg for 3 consecutive days. Seventeen patients had hepatosplenomegaly, two of these had ascites. Three patients had diffuse colonic polyposis, one of these had ascites. Except for one who developed mild hematemesis 3 days after treatment, all patients tolerated the drug very well. However, 11 patients developed a fever 24 to 48 hours after completing treatment, which lasted for 2–3 days and coincided with increased excretion of schistosomal antigens in urine. Three months after completing therapy, all except one young patient ceased to have live eggs in the stools or rectal tissue. Six months after treatment, three patients with colonic polyposis showed marked clinical improvement and sigmoidoscopic and barium enema examination demonstrated almost complete disappearance of all polyps.