In 105 patients with active infective endocarditis, disease-associated complications defined as severe heart failure (New York Heart Association class IV), embolic events and in-hospital death were correlated to the vegetation size determined by both transthoracic and transesophageal echocardiography. A detailed comparison between anatomic and echocardiographic findings, performed in a subgroup of 80 patients undergoing surgery or necropsy, revealed that true valvular vegetations can be reliably identified by echocardiography in the vast majority of patients; the detection rate was significantly higher for the transesophageal (90%) than for the transthoracic (58%) approach, particularly when infected prosthetic valves were evaluated. However, an accurate echocardiographic differentiation between true vegetations and other endocarditis-induced valve destruction (ruptured leaflets or chordae) is impossible. The correlation of vegetation size with endocarditis-associated complications showed that patients with a vegetation diameter greater than 10 mm had a significantly higher incidence of embolic events than did those with a vegetation diameter less than or equal to 10 mm (22 of 47 versus 11 of 58; p less than 0.01). Particularly for patients with mitral valve endocarditis, a vegetation diameter greater than 10 mm was highly sensitive in identifying patients at risk for embolic events. Vegetation size, however, was not significantly different in patients with and without severe heart failure or in patients surviving or dying during acute endocarditis. In addition, no significant correlation was found between vegetation size and location of endocarditis or type of infective organism.(ABSTRACT TRUNCATED AT 250 WORDS)