Analysis of 99 M-mode echocardiograms recorded during and up to 144 months after healing of active bacterial endocarditis limited to the mitral valve in 27 patients disclosed the following: (1) Little to no change occurred in the echocardiographic size of the vegetations during the first 6 weeks after diagnosis and institution of appropriate antibiotic therapy unless a major systemic embolus occurred. (2) The echocardiographic size of the vegetations did not determine the amount of cardiac damage or dysfunction produced by the valvular infection. (3) The larger the vegetations by echocardiogram, the greater was the likelihood of a clinical event compatible with a systemic embolus. (4) The gravest prognostic sign yielded by the echocardiogram was evidence of rupture of chordae tendineae. (5) Although a useful adjunct to diagnosis before appropriate antibiotic therapy was instituted, once bacteriologic cure was achieved, the echocardiogram was of limited value in delineating an active from a heated vegetation. (6) The echocardiographic appearance of the vegetations was not determined by the type of infecting bacterium.