The efficacy of M-mode echocardiography in identifying vegetations and the clinical relevance of such a finding were evaluated in 36 patients with bacterial endocarditis, 18 of whom had pre-existing valvar lesions. Of 22 patients with vegetations demonstrated by echocardiography, 21 had major complications of bacterial endocarditis and eight died, whereas only eight of the 14 patients without detectable vegetations had similar complications and only two died. Nine of the 11 patients with pre-existing valvar lesions and echocardiographic evidence of vegetations came to operation or necropsy, and in eight of these the presence of vegetations was confirmed. In the other, none was found at surgery performed after an interval of six months. M-mode echocardiography can reliably detect vegetations in patients with bacterial endocarditis even in the presence of pre-existing valvar lesions, and may permit the identification of a subset of high risk patients who may need early surgery.