Forty-one patients in whom the diagnosis of a non-stenotic bicuspid aortic valve had been established by noninvasive techniques were followed up for a mean of 10.9 years. During this period, 2 patients required aortic valve replacement because of the development of calcific aortic valve stenosis at the ages of 52 and 64 and 5 others developed evidence of mild aortic valve stenosis. The appearance of calcium in a bicuspid aortic valve suggests the possibility of subsequent calcific aortic stenosis, and patients with this feature should be carefully followed up. Bacterial endocarditis on the aortic valve occurred in 3 patients, one of whom developed severe aortic regurgitation and subsequently died. Patients with a bicuspid aortic valve are at definite risk from bacterial endocarditis and should receive appropriate antibiotic prophylaxis. In 26/41 (63%) patients there was no clinical change during the follow-up period, including 7 of the patients over the age of 50.