The effectiveness of anti-parasite treatment with benznidazole in the chronic Chagas disease (ChD) remains uncertain. We evaluated, using data from the NIH-sponsored SaMi-Trop prospective cohort study, if previous treatment with benznidazole is associated with lower mortality, less advanced cardiac disease and lower parasitemia in patients with chronic ChD. The study enrolled 1,959 ChD patients and abnormal electrocardiogram (ECG) from in 21 remote towns in Brazil. A total of 1,813 patients were evaluated at baseline and after two years of follow-up. Those who received at least one course of benznidazole were classified as treated group (TrG = 493) and those who were never treated as control group (CG = 1,320). The primary outcome was death after two-year follow-up; the secondary outcomes were presence at the baseline of major ChD-associated ECG abnormalities, NT-ProBNP levels suggestive of heart failure, and PCR positivity. Mortality after two years was 6.3%; it was lower in the TrG (2.8%) than the CG (7.6%); adjusted OR: 0.37 (95%CI: 0.21;0.63). The ECG abnormalities typical for ChD and high age-adjusted NT-ProBNP levels suggestive of heart failure were lower in the TrG than the CG, OR: 0.35 [CI: 0.23;0.53]. The TrG had significantly lower rates of PCR positivity, OR: 0.35 [CI: 0.27;0.45]. Patients previously treated with benznidazole had significantly reduced parasitemia, a lower prevalence of markers of severe cardiomyopathy, and lower mortality after two years of follow-up. If used in the early phases, benznidazole treatment may improve clinical and parasitological outcomes in patients with chronic ChD. ClinicalTrials.gov, Trial registration: NCT02646943. Chagas disease remains as one of the most neglected infectious diseases of the world, with millions of persons infected and dozens of thousands of deaths every year. The disease has a very long natural history and few options of etiologic treatment, including antiparasitic [1] drugs benznidazole and nifurtimox. Benznidazole has been recommended for treatment of the disease based mostly in physiopathological assumptions and relatively small observational studies and, recently, a negative trial was published, showing the absence of benefit of benznidazole in patients with Chagas cardiopathy. Thus, there is an urgent need not only of new therapeutic options but also of further evidence of the utility of benznidazole, at least in those patients with less severe disease. We conducted an observational study in the NIH SaMi-Trop cohort comparing patients with self-reported previous treatment with benznidazole with non-treated ones, using as primary outcome death in the follow-up of 2 years, and as secondary outcomes markers of severity based in the ECG and NT-proBNP levels, as well as parasitemia by PCR for Trypanosoma cruzi DNA in blood. Patients previously treated with benznidazole had significantly reduced parasitemia, a lower frequency of markers of severe cardiomyopathy and reduced mortality. If used in the early phases of Chagas disease, benznidazole treatment may result in better clinical and parasitological outcomes. It is reasonable to consider treating all Chagas disease patients without advanced cardiopathy with benznidazole, especially those less than 50 years-old of age.