Ten patients with the billowing mitral leaflet syndrome complicated by infective endocarditis are reported. Two patients had a non-ejection systolic click and 8 had both a non-ejection systolic click and a late systolic murmur. These auscultatory features were difficult to detect in 4 instances in that they were intermittent, soft, or brought out only with postural change. Seven patients were unaware of their cardiac lesions. A low grade pyrexia was present in all 10 patients. Four patients presented with clinical features caused by reversible neurological lesions. Blood cultures were positive in all patients, with Staphylococcus albus the infecting organism in 6. Antibiotic therapy was successful with significant mitral regurgitation supervening in only one instance. The importance of the billowing leaflet as a potential site of infective endocarditis is emphasized. It seems that antibiotic prophylaxis is indicated at times of increased risk of infection in subjects with a non-ejection systolic click or a late systolic murmur.