The evolution of Chagas' cardiomyopathy is poorly understood. We therefore examined the development of cardiac lesions in a rural Brazilian community for a period of 7 years. Initially, 42% of 1017 residents were seropositive for infection with Trypanosoma cruzi. Age-specific infection rates indicated that most had become infected before the age of 20 years. On follow-up, it appeared that those persons who developed cardiac lesions did so soon after infection, since the incidence of right bundle branch block and other ventricular conduction defects (VCDs) was also highest before age 20 years. The progressive nature of these lesions was demonstrated by frequent development of additional electrocardiographic abnormalities and high mortality among infected adults with VCDs. In contrast, mortality was low and approximately the same for seropositive and seronegative adults under 60 years who had normal electrocardiograms. Electrocardiography during the early asymptomatic stage of infection was able to distinguish persons with potentially lethal cardiac lesions from those with a benign prognosis.