We examined 129 standard M-mode echocardiograms obtained in 65 patients (16 to 73 years old) with bacterial endocarditis. Twenty of the 22 patients with vegetations recognizable by echocargiography died, or underwent cardiac operation (mean interval from admission 22 days, and range two to 120 days). Vegetations were seen on the echocardiograms in 22 (aortic 10, mitral nine and tricuspid three, with anatomic confirmation in 19). Of patients without vegetations on echocardiography none underwent emergency operation or died as a result of cardiac disease (mean follow-up period of 14 months, range of two to 38 months). Other echocardiographic findings in those with vegetations included early mitral-valve closure (six), "flail" aortic leaflet (three), and "flail" mitral leaflet (three). Echocardiography can provide a rapid, reliable noninvasive diagnosis of bacterial vegetations in certain patients with bacterial endocarditis and may identify patients with more severe disease who may require operative intervention.