The infrequent occurrence of mural thrombi in the chambers of the heart is noted. The relevant literature is reviewed. The findings in eighty-two cases of bacterial endocarditis superimposed on rheumatic heart disease and in 47 examples of bacterial endocarditis without underlying rheumatic valvulitis are reported. These findings may reflect the infrequency of auricular fibrillation and of severe mitral stenosis in cases of bacterial endocarditis. The results further substantiate the origin of emboli from the valvular vegetations, bacterial or nonbacterial, rather than from the cardiac chambers, and this correlates with the bland character of most of the infarcts.