Twenty five patients with chronic Chagas' disease were selected presenting positive serology and high parasitaemia. A group of fifteen partients was treated with Nitrofurfurilidene during 120 days with daily doses of 8-10 mg per kg of body weight. The remaining group of ten individuais received only placebo. Yatrogenic alteration of liver, hematopoiesis or kidney were not obseived during or after treatment. Xenodiagnosis of the experimental 15 patient group was negative at the 60th day of treatment and remained so up to 5 years that the observation lasted. Regarding the serum diagnosis the behaviour was not alike eventhough the experimental group presented lower levels of antibodies after the 12th month of the treatment. The authors discuss the disagreement between the xenodiagnosis and serologie results and offer an hypothesis to justify it. They conclude that Nitrofurfurilidene has some efficiency in the chronic phase of Chagas' disease although less evident than in the acute form.