A cross-section study, type "case-control" was performed in four endemic areas for Chagas' disease in Brazil, two of them in Minas Gerais State, one in the State of Piauíand the other in the State of Paraíba. A total of 716 pairs of persons with same age and sex, one with a positive serology and the other with a negative serology for Chagas' disease were included in the study. This kind of study was performed to determine the chagasic component responsible for the morbidity of the disease in different areas of the country. The clinical symptoms and signs and the eletrocardiographic changes between the serologic positive and serologic negative groups studied in 264 pairs of persons in the district of Iguatama-Pains and 274 in Virgem da Lapa, both in Minas Gerais State, when compared with the same parametres applied in 109 pairs studied in the localities of Colônia and Oitis in the district of Oeiras, State of Piauí and 69 in the districts of Aguiar and Boqueirão dos Cochos in the State of Paraíba, both in northeast of Brazil, showed a marked difference in the morbidity of the disease in different areas. In the areas of Minas Gerais State, although the natural transmission of Chagas' disease had been interrupted since 15 years ago in Iguatama-Pains and 5 years ago in Virgem da Lapa, the cardiac morbidity by the chagasic component was approximately of 30% at he moment of the study. In the areas of Piauí and Paraíba States, although the natural transmission of Chagas' disease is still active, the cardiac morbidity by the chagasic component was lower than 15 and 10%, respectively.