BACKGROUND AND AIM OF THE STUDY: Although, in small aortic annulus or aortic annulus calcification, it is recommended that valves are implanted in a tilted position, mechanical valves show impaired hemodynamic performance when positioned in this way. To date, no investigations have been conducted with biological valves implanted in a tilted position. METHODS: Measurements were performed in a pulsatile flow simulator. The aortic roots were mounted in a fluid reservoir and tested with bioprosthetic valves implanted in the regular position (21 mm; n = 7) or at a 200 tilt (23 mm; n = 7). Additional 21 mm valves were implanted in both positions with a systemic pressure of 120/80 mmHg. Subsequently, the valves were implanted into a glass model and flow visualization monitored by adding air bubbles illuminated with a laser diode. RESULTS: The 21 mm valves showed a slightly higher transvalvular gradient in the tilted than in the regular position, while 23 mm valves in a tilted position showed a lower gradient than 21 mm valves in the regular position. Flow in the regular position was seen to be straight and central, but in the tilted position was diverted and impacted on the aortic wall. Vortex formation in the tilted position was more pronounced than in the regular position, with certain low-flow areas being observed. CONCLUSION: The implantation of a one size-larger bioprosthetic valve at a 20 degree tilt in a small aortic root resulted in a slight reduction in transvalvular gradient compared to a smaller valve implanted in the regular position. Whilst mechanical valve performance is markedly compromised in the tilted position, the bioprosthetic valve showed only minor impairment of transvalvular pressure gradient due to tilting, and this was overcompensated by the larger valve size. However, the advantage of a greater orifice area must be traded against the consequences of the observed flow disturbances.