OBJECTIVE--To determine the role of exertion as a precipitating factor in syncope in patients with aortic stenosis. DESIGN--Follow up for at least one year after aortic valve replacement. SETTING--Regional cardiac centre. PATIENTS--Consecutive patients who underwent aortic valve replacement for aortic stenosis and who had a history of syncope and no other clinically significant cardiac lesion were recruited during a five year period. METHODS--Patients were interviewed to determine the characteristics of syncope and were divided into those in whom syncope was consistently related to exertion (group 1, 26 patients) and those in whom it was not (group 2, 13 patients). Recurrence of syncope after valve replacement was determined at visits to the clinic. RESULTS--Groups 1 and 2 did not differ significantly with respect to valve area, age, or sex. In 13 of the 26 patients in group 1 syncope was preceded by angina despite the absence of coronary artery disease, but none of the 13 patients in group 2 had angina. After valve replacement none of group 1 and eight of the 13 patients in group 2 had recurrence of syncope (p < 0.01). CONCLUSION--When patients with aortic stenosis experience syncope unrelated to exertion, the symptom is usually unrelated to the valve disease and another cause should be sought.