A recent survey of 3,500 consecutive autopsies at the Institute of Pathology of the University Hospitals of Cleveland revealed 8 cases of acquired bicuspid aortic valve. This paper comprises a study of these cases, with especial reference to the cause and significance of the acquired bicuspid aortic valve. REPORT OF CASES Case 1. —A 41 year old white man had clinical diagnoses of endocarditis lenta and aortic stenosis and insufficiency. There was no history of rheumatic fever, although the patient was told by a physician thirteen years prior to admission that he had heart disease. The heart was enlarged and revealed a systolic thrill at the base, a loud, harsh systolic murmur and a diastolic murmur over the aortic area and a systolic murmur at the apex. The heart weighed 490 Gm. There were coarse fibrous adhesions between the aorta and the pulmonary artery, in the atrioventricular sulcus and between