We analyzed 50 cases of bicuspid aortic valve endocarditis in patients who presented to St. Thomas' Hospital from 1970 through 1998. These represented 12.3% of the 408 cases of native valve endocarditis (NVE). All patients were male, and their mean age was 39 years. Forty-five of the 50 cases were pathologically proven; 47 were clinically definite according to the Duke criteria and 49 according to our modifications of the Duke criteria. Viridans streptococci and staphylococci accounted for 72% of cases. The prevalences of clinical features were similar to those seen in NVE: fever (temperature >/=38 degrees C, 74%) and malaise (70%), although dyspnea was more frequent (36%). There was a high incidence of serious complications (72% heart failure; 30% periannular abscesses). Surgery was required during the initial admission in 82% of cases. Overall mortality was 14%, and surgical mortality was 9%. Few patients knew they had a "heart condition," and a bicuspid aortic valve was detected in only 35% of echocardiograms performed before surgery.