Among 71 patients who underwent aortic valve replacement with a Björk-Shiley prosthesis, 13 (18.3%) had concomitant enlargement of the aortic annulus at the noncoronary cusp. Mean age of the 13 patients at operation was 34.4 years. Nine of them had combined mitral stenosis. One patient died in the early postoperative period, for an operative mortality of 7.7%. The aortic annulus before enlargement was 20.9 mm and after enlargement, 24.3 mm on the average. The net increase in diameter of the aortic annulus was 3.3 mm, which was statistically significant (p < 0.005). The tissue diameter of the prosthesis was 1.2 mm larger on the average than the aortic annulus diameter before enlargement, and a prosthesis at least one size larger could be implanted in the narrow aortic annulus.