One hundred twenty-nine consecutive patients underwent isolated aortic valve replacement with the Hancock porcine xenograft between July, 1974, and December, 1976. The hospital mortality rate was 3.9 percent. No patient was treated with anticoagulants, and valve-related complications were extremely rare. The smaller prosthetic sizes (19 and 21 mm. stent diameter) should be used with extreme caution, and the 19 mm. prosthesis should probably never be used in the audult patient. Two methods of managing the small aortic root are emphasized: one to avoid using the smaller prosthetic size in adults and the other to alter greatly the root size in children who have a hypoplastic aortic annulus. Acceptable calculated orifice sizes and left ventricular--aortic (LV-Ao) pressure gradients may be obtained with the 23 mm. or larger prostheses. Actuarial survival curves show 92 percent of patients alive and well at 24 months' follow-up.