Trypanosoma cruzi, the agent of Chagas disease, is genetically classified into two major evolutionary lineages, T. cruzi I and T. cruzi II. In Southern American Cone countries it is T cruzi II which causes most cases of severe chronic Chagas disease. Contrary to this, we isolated T. cruzi I nested in endomyocardial biopsies of a chronic chagasic patient with end-stage heart failure. Our finding should alert clinicians to the possibility of severe Chagas disease in all regions where T. cruzi circulates, regardless of its lineage.