In a prospective clinical trial, patients with urinary schistosomiasis from the Sudan and from other African countries were treated with two or three doses of metrifonate (10 mg/kg), respectively, in two weeks' intervals. Patients seen in the Sudan (n = 37) were followed up for 3 months, those seen in Hamburg (n = 17) up to 20 months. Each dose of metrifonate led to a decrease of almost 90% in egg excretion independently of the pretreatment intensity of infection so that after three doses a reduction of almost 99.9% was achieved. The estimated proportion of parasitological cure after two doses of metrifonate was in the order of 24% in the lightly infected patients seen in Hamburg, and in the order of 8% in the heavily infected Sudanese patients. Almost 59% of the patients who received a complete metrifonate treatment of three doses stopped to excrete ova of S. haematobium. Drug failure was found in almost the same proportion in the two patient groups (17.5% and 16.2%, respectively) and could not be overcome by additional doses of metrifonate. The considerable reduction in egg excretion after a single dose of metrifonate might have a remarkable benefit in large scale programmes where the aim is drastic reduction of worm burden rather than complete cure.