One hundred sixteen male patients with uncomplicated Schistosoma mansoni infection were randomized to treatment with oxamniquine (20 mg/kg a day for 3 days) or niridazole (15 mg/kg a day for 8 days) in an in-hospital study. Fifty-four patients were treated with oxamniquine; no serious drug side effects were noted. Twenty-six percent of children 16 years old and younger were cured, while 85% of adults were cured. Egg reduction was 89%. Sixty-two patients were treated with niridazole; four patients (one child and three adults) had serious side effects which necessitated discontinuing the drug. Seventy-one percent of children and 84% of adults were cured. Egg reduction was 84%. Minor clinical side effects occurred during therapy with both drugs (26% with oxamniquine, 48% with niridazole) but cleared after completion of therapy. Niridazole is the more effective drug for the child with uncomplicated Schistosomiasis mansoni, but oxamniquine is advised for the adult with this disease.