Diseased vessels experience a significantly different blomechanical environment than healthy vessels due to the presence of transitional and turbulent flow, which may be responsible for cell damage or plaque disruption. With an objective of investigating turbulent characteristics in a diseased vessel, direct numerical simulations were conducted based on a patient-specific carotid bifurcation with a severe stenosis under pulsatile flow conditions. Computerized tomography images and color Doppler ultrasound measurements were acquired to obtain the anatomical geometry and in vivo flow waveform, respectively. The spectral element method, which is ideally suited for transitional and turbulent flow simulation, was employed in the numerical technique. Transition to turbulent flow with chaotic velocity fluctuations triggered by Kelvin-Helmholtz instabilities in the internal carotid artery was observed in the systolic phase. Turbulence intensities were significantly enhanced downstream of the stenosis during the deceleration phase of systole. The interaction of two strong shear layers generated from the bifurcation apex and distal end of stenosis as well as flow separation due to complex irregular morphology, created large WSS spatial gradients in the stenosis and high WSS magnitudes (250-450 dyne/cm2). Turbulent pressure fluctuations downstream of the stenosis and a large transient pressure drop occurring over the stenosis region will likely produce significant dynamic stresses-cyclic loading on plaques. Turbulent velocity and pressure fluctuations in the post-stenotic region were observed in audible frequency range of 100-300 Hz. This study demonstrates the hemodynamic environment in a diseased carotid bifurcation to be extremely complex during systolic phase and significantly different than that of a healthy carotid bifurcation.