During systole a small portion of the mainstream aortic flow is intercepted by the sinus ridge, or downstream corner of the sinus of Valsalva. This fluid curls back toward the ventricle to form a large eddy, or vortex, that spins within the sinus cavity. The fluid motion within similar recirculating flows is known to be unstable with an early transition to turbulence. The stability of the aortic sinus vortex was examined in the current study in an in vitro pulsatile flow rig. The geometry of the experimental test section was the same as the geometry of the natural aortic root. Different model valves, including a natural valve, were placed in the test section, and different flow conditions were studied. Point velocities were measured by hot film probes placed at two locations within the sinus vortex. The velocity waveforms and their power spectra were used to determine the stability of the sinus flow. The experimental results revealed that the aortic sinus vortex becomes turbulent under simulated exercise conditions. Turbulent intensities were highest near the sinus ridge, which is the location of the coronary ostia. Despite the transition to turbulence within the vortex, the mainstream aortic flow upstream from the valve remained laminar. The turbulence within the vortex was also associated with vibration of the valve leaflets under exercise conditions. These vibrations may be related to the systolic ejection murmurs that are heard clinically. Furthermore, the localized turbulence may explain the location of atherosclerotic lesions and dissecting aneurysms, as well as the distribution of the lesions of bacterial endocarditis.