The relationship between anatomical configuration of collateral flow and the outcome of the internal carotid artery (ICA) occlusion was analyzed. Sixty-one patients with occlusion of ICA were studied with transcranial Doppler ultrasonography. The authors monitored blood flow velocities in the middle cerebral artery (MCA) ipsilateral to the ICA occlusion during a series of carotid compressions. Blood flow through three major collateral arteries (anterior and posterior communicating arteries and ophthalmic artery) was determined as a residual flow in MCA after each compression. Twelve subjects had asymptomatic occlusion and 46 had stroke; patients with stroke were divided into mild, moderate, and severe stroke groups. Subjects with asymptomatic occlusion had a higher number of patent collateral vessels than severe stroke patients (p < 0.001). The same relationship was found between mild and severe stroke patients (p < 0.001) and between moderate and severe stroke (p < 0.01). Patients with watershed ischemia had a lower number of patent collateral arteries than patients with thromboembolic ischemia as revealed by CT examination (p < 0.02). This study demonstrates the crucial role of collateral flow in the stroke pathogenesis and stroke outcome in subjects with extracranial ICA occlusions.