Analysis of 37 M-mode echocardiograms recorded during the period of active bacterial endocarditis (ABE) involving the aortic valve (AV) in 17 patients disclosed one or more echocardiographic abnormalities involving the aortic valve cusps or their immediate vicinity in 15 (88%), including "shaggy" echoes indicative of vegetations in 12 (71%). Of the 12 patients with echocardiographic evidence of AV vegetation, 11 developed overt congestive heart failure (CHF) and either died or had AV replacement, and seven had clinical events compatible with systemic emboli; of the five patients without echo-demonstrated vegetations, only one had CHF, none had AV replacement, two died, and one had a systemic embolus. In comparison to our previously reported echocardiographic observations in patients with ABE involving either the mitral (29 patients) or tricuspid valve (23 patients), infection involving the AV was far more liable to produce overt CHF and systemic emboli, to necessitate valve replacement, and to cause death during the period of active infection.