From 1982 to March 1999, 276 St. Jude Medical prostheses were implanted in aortic position. Of the 276 patients, 6 (2.2%) required redo aortic valve replacement due to aortic stenosis. The peak velocity measured by continuous-wave Doppler echocardiography ranged from 3.5 to 5.4m/sec with mean of 4.55m/sec. Aortic stenosis was attributable to pannus formation in 3 patients, valve thrombosis in 1 patient, and prosthesis-patient mismatch in 2 patients. The prostheses of patients with pannus formation were implanted in valve orientation parallel to the septum. It is therefore considered that the St. Jude Medical prosthesis should be implanted perpendicular to the septum in the aortic position and that careful follow-up observation of the patients should be made, particularly with echocardiography.