Right ventricular endomyocardial biopsy was used to study myocardial involvement in 42 patients with chronic Chagas' disease. Patients were divided into three groups: group A included 16 patients with normal ECGs, normal chest x-rays, and no symptoms; group B included 15 patients with abnormal ECGs and no cardiomegaly; and group C included 11 patients with abnormal ECGs and cardiomegaly. Biopsy fragments were analyzed for hypertrophy, degeneration of myocardial fibers, and interstitial changes such as edema, fibrosis, and inflammatory infiltrate. In group A, 5 of 16 biopsies exhibited none of the previously mentioned alterations. The frequencies pathologic alterations in groups A, B, and C, respectively, were: hypertrophy 31%, 66%, and 100%; degeneration 50%, 86%, and 81%; edema 43%, 46%, and 36%; fibrosis 12%, 33%, and 54%; and inflammatory infiltrate 37%, 66%, and 65%. These data suggest that myocardial lesions of Chagas' disease represent a continuous progression from fiber destruction to substitution by fibrosis, with compensatory hypertrophy; these data also suggest that cardiac dilatation occurs when the extent of fibrosis no longer allows for efficient compensatory hypertrophy.