Light and electronmicroscopical studies were performed on myocardial samples, obtained from the atrium, papillary muscle, anterior wall of left ventricle and the operated valves. Degenerative verrucous endocardiosis and bacterial endocarditis were encountered but no histological signs of rheumatic activity were found. No specific diagnostic electronmicroscopical findings were present and the ultra-structural changes present included alterations in the mitochondria, an increase in the number of fat droplets, a focal increase in glycogen and focal muscle disruption and necrosis. The detection of anti-heart antibodies by indirect immunofluorescent studies and their possible relation to myocardial damage are discussed. In addition, the importance of myocardial changes, the presence of small myocardial scars and the increase in interstitial collagen tissue are reviewed in relation to myocardial function and it is suggested that they can be used as a prognostic index of myocardial function. It is also suggested that earlier operation of diseased valves be performed in order to prevent further myocardial damage and impairment of myocardial function.