Transcranial color-coded sonography (TCCS) was used to assess primary willisian collaterals in 100 patients with extracranial internal carotid artery (ICA) stenosis. Their importance was determined during carotid endarterectomy (CEA) by transcranial Doppler measurement of blood flow velocity in the ipsilateral middle cerebral artery (MCAV) before and after carotid clamping. All patients had unilateral ICA disease of at least 60% stenosis. Twenty-nine ICAs (14.5%) were occluded, 70 vessels (35%) were stenosed by 80 to 99%, 43 vessels (21.5%) were stenosed by 60 to 79%, and 53 ICAs had stenosis of less than 60%. Temporal hyperostosis precluded TCCS in 15 patients (15%). Anterior cerebral/communicating artery collaterals were detected in 40 patients (49%) and posterior cerebral/communicating artery collaterals were detected in 22 patients (27%). No patients with ICA stenosis of less than 80% had established collateral pathways. Patients with willisian collaterals showed higher postclamp MCAVs as a proportion of the preclamp value during CEA (72% [62-81]; median with 95% confidence interval) than did those without primary collaterals (46% [34-58], p = 0.02). TCCS allows noninvasive assessment of intracranial primary collaterals whose functional importance is recognized during abrupt hemodynamic challenge. It may determine which patients are at greatest risk of ischemia during cerebral revascularization.