Observations are described on the fibrillar nature and structural features of the collagenous interstices of the human myocardium in chronic chagasic cardiomyopathy, comparing them with similar observations in normal hearts, using the picrosirius red technique and polarization microscopy. A total of 28 adult hearts obtained at autopsy were used: 5 control without evidence of cardiac disease, and 23 with chronic Chagas' heart disease. The findings in the myocardium of those without Chagas' disease were in keeping with those reported in the literature. In those with Chagas' disease diffuse interstitial fibrosis could be observed in all cases, albeit to varying degree. The pattern was that of a diffuse increase in the amount of thick collagen fibers surrounding bundles of muscle fibers (perimysial matrix), varying in intensity from one area to another, and around the intramyocardial coronary vessels, combined with a less pronounced increase in the matrix of endomysial collagen. The relationship between the observed myocardial fibrosis and cardiac function, and the potential mechanisms for its production, are discussed. Further research is needed into the patterns and pathogenesis of myocardial fibrosis in order possibilities for prevention and development of corrective forms of therapy for the fibrotic process.