In 15 patients with internal carotid artery (ICA) dissections, patterns of brain infarctions visible on CT were categorized according to a pathogenetically oriented classification system. This differentiated ischemic brain damage due to low flow from thromboembolic infarctions. Simultaneously, cerebral hemodynamic reserve was evaluated by means of both CO2-dependent vasomotor reactivity and HMPAO- and 99mTc-RBC-SPECT. Six out of 11 patients with ischemic infarctions had the territorial type of brain lesion, suggesting distal embolism. Five patients revealed the pattern of hemodynamically induced low-flow infarctions. Cerebral hemodynamic reserve was significantly decreased only in these latter patients. In ICA dissections the frequencies of hemodynamically and thromboembolically induced brain infarctions is approximately equal. This finding suggests that only half of the patients with symptoms of symptomatic dissections had brain emboli, whereas in the other half the dissections lead to a low-flow effect in terminal supply areas which is so severe that non-embolic brain infarctions occur. This differentiation can help decide whether antithrombotic or hemodynamic treatment should be given.