Studying the mitral valve in chronic Chagas' heart disease, searching a possible cause effect relationship between this condition and valve prolapse. Seventeen hearts were surveyed from individuals exhibiting chronic chagasic cardiopathy, 11 males and 6 females, aged between 31-84 (average 54) years. The hearts came from necropsies carried out until a year before. Properly preserved samples were analyzed macro and microscopically. Fragments were excised from the mitral valve papillary muscles, anterior, lateral and posterior regions of the left ventricle, interventricular septum, free wall of right ventricle and atrium walls. Chronic myocarditis was found ill all the samples (mild in 6 of them, moderate in 7 and severe in 4). Also, the papillary muscles exhibited myocarditis in all of the samples, and in 15 of them the degree of severity was equal to or superior than the observed in degree other regions of the heart. The chronic chagasic cardiopathy, the papillary muscle constitute a frequent site of myocarditis. This fact must be held in mind when one discusses the pathogenesis of the prolapse of the valve.5891-4