Aortic root abscess is frequently a lethal complication of infective endocarditis. Early diagnosis of this complication is of paramount importance because antibiotic therapy is often ineffective and early surgery is probably the treatment of choice. We have compared the diagnostic accuracy of cross-sectional echocardiography with operative findings in the diagnosis of aortic root abscess. Aortic root abscess was diagnosed in nine of 129 patients with infective endocarditis at the National Heart Hospital between 1983 and 1985. Cross-sectional echocardiography demonstrated the presence and location of the aortic root abscess in eight cases; in the ninth case a small abscess was missed. In two other cases, a large abscess was visualised, but abscesses of between 2 mm and 4 mm were missed; the extent and size of large aortic abscesses tended to be underestimated. Echocardiography should be an integral part of the investigation of patients with aortic valve infective endocarditis.