Seventy seven patientes with Chagas disease were treated with nifurtimox. They were divided into 4 groups. Group I consisted of 30 patients in the acute phase of the disease. Twenty two patients took an insufficient dose. Only 8 patients used the drug for 120 days in an initial daity dose of 15 mg/kilogram body weight for up to 2 weeks and later in a daily dose of 10 mg/kilo body weight. In 4 patients (50%) the xenodiagnosis became negativa after treatment and in 3 the Machado Guerreiro test became negative. One patient died with acute cardiac failure on the 5th day of treatment. Six patients developed evidence of sensory peripheral neuritis while on treatment. Group II consisted of 15 outpatients in the chronic phase. They also took the drug for 120 days in a variable dosage of 5-15 mg/kilo body weight. They had at least one xenodiagnosis performedafter treatment in four patients xenodiagnosis became negative. The treatment had no effect on the electrocardiogram, degree of cardiomegaly, or appearance on barium swallow. Treatment did not prevent one patient from developing an arrhythmia and another cardiac failure. Group III also contained 15 patients in the chronic phase with positive xenodiagnosis treated with 10 mg/kilo body weight for 120 days. Twelve serial monthly xenodiagnosis were done after treatment using 80 T. infestans (10 to a box) for each examination. In only 28.5% of the patients were all these xenodiagnoses negative. The frequency of parasitemia was decreased in the other patients but not eradicated. The number of xenodiagnoses positive before treatment was 43% and afterwards 24.4%. The number of positive boxes diminished from 29.6% to 7%. Group IV consisted of 17 patients in the chronic phase with positive xenodiagnosis treated with 8 mg/kg per day. Eight were treated with this dose for 120 days, 5 for 100 days and 4 for 60 days. The percentage of patients with 12 serial xenodiagnoses negative after treatment was 56.25%. A reduction in the incidence of positive xenodiagnoses from 76.5 to 10.3% also ocurred in this group and the number of positive boxes diminished from 28.6 to 3.1%. There is no advantage in prolonging treatment beyond 2 months. The percentage of cure is much less than that obtained in Argentina, Chile or Rio Grande do Sul (Brazil). The drug produces many side effects especially a peripheral neuritis.