46 subjects with obstructive carotid artery disease were investigated with transcranial Doppler ultrasonography. Their baseline blood velocities (V) in the middle, anterior and posterior cerebral artery (MCA, ACA and PCA) and in the extracranial internal carotid artery (ICA) were measured and the pulsatility index (PI) calculated for each vessel. Thereafter the vasomotor reserve in both MCAs was tested. Typical patterns of V, PI and vasomotor reactivity are presented. Arterial collaterals were recognized by their relatively increased velocities. We demonstrated a close association of the baseline variables V and PI and the total vasomotor reactivity (hypocapnic plus no, hypercapnic response) by calculating an index of Uhem related to the cerebrovascular tone. The Uhem index is expressed by: Uhem index = VMCA.PIMCA/VPCA.PIPCA The relationship between Uhem index and the total vasomotor reactivity seemed to correspond to a hyperbolic curve. The hyperbolic tangent of Uhem index and total vasomotor reactivity correlated highly significantly, r = 0.8203, p or = 0.94 indicated normal total cerebral vasomotor reactivity in contrast to an impaired reactivity when < or = 0.81. Findings in 20 patients investigated post hoc supported the validity of our concept.