The evidence for subendocardial ischaemia was studied in 12 patients with discrete subvalvar aortic stenosis. Symptomatology, electrocardiographic criteria, and pressure difference across the left ventricular outflow tract were compared with the subendocardial flow index (diastolic pressure time index systolic pressure time index). All symptomatic patients had a large pressure difference and abnormal index, but 4 asymptomatic patients had pressure differences greater than 60 mmHg and a low index. One of these 4 patients had a normal resting electrocardiogram. In patients with borderline accepted indications for surgery, calculation of the subendocardial flow index may be an additional useful variable in the timing of surgery.