Mononuclear cellular infiltrates and extensive fibrosis, with or without apical ventricular aneurysms, are the usual morphological findings in chronic chagasic cardiomyopathy. These lesions are thought to be mediated by immune phenomena rather than by continuing parasitic invasion of the heart. In the present report, we correlated clinical, immunohistochemical and ultrastructural findings in 30 endomyocardial biopsies from patients with chronic chagasic cardiomyopathy. In 12 of these biopsies, immunocytochemical techniques were used to identify and count leukocytes (common leukocyte antigen, CLA), T lymphocytes (UCHL-1 antibody) and B lymphocytes (L-26 antibody). The biopsy specimens showed variable degrees of myocardial hypertrophy and mononuclear infiltrates. No tissue forms of trypanosomes were found. The endocardium averaged 24 +/- 12.6 microns (mean +/- SD) in thickness. The mean myocyte diameter was 20 +/- 7.33 microns. The hearts were severely fibrotic containing a mean of 24.1 +/- 12.8% of fibrous tissue (range 8.2-49%), mast cells were scarce. Mononuclear cell infiltrates were found in 25 of the 30 biopsies. In 12 biopsies, immunohistochemical studies showed that the majority of the lymphocytes were T lymphocytes and associated with necrotic or degenerating myocytes. 10 of the 12 biopsy samples showed 5 or more CLA-positive mononuclear cells/high power field. In these 10 patients, T and B lymphocytes represented 32 and 13% of the total mononuclear infiltrating cells, respectively. The remaining cells were monocytes and macrophages.