Transcranial Doppler is often employed for the assessment of cerebral autoregulation. The study by Asil et al. [Asil T, Utku U, Balci K, Uzunca I. Changing cerebral blood flow velocity by transcranial Doppler during head up tilt in patients with diabetes mellitus. Clin Neurol Neurosurg 2007;109:1-6] raises some important issues regarding the better understanding of the influence of autonomic nervous system in cerebral autoregulation. We suggest including some additional parameters, especially carbon dioxide levels, transcranial Doppler waveform patterns and their characteristics to further elucidate cerebral hemodynamics and the mechanisms of cerebral autoregulation. Vasomotor reactivity testing under controlled circumstances may add to the quantification of cerebral autoregulation.