The authors examined 61 subjects with carotid angiography and 50 with vertebral angiography. Angiograms were evaluated for collateral flow through the ophthalmic, anterior communicating, and posterior communicating arteries. The authors evaluated the patency of collateral vessels directly using transcranial Doppler ultrasonography; they made indirect detection after the compression of carotid and vertebral arteries while monitoring flow velocities in the middle cerebral artery. They established criteria for the hemodynamic significance of tested collateral vessels. A combination of carotid compressions and transcranial Doppler ultrasonography detected the patency of the ophthalmic and anterior communicating arteries with a specificity and sensitivity of 1.00. Examination of the posterior communicating artery had a sensitivity of 0.97 and specificity of 0.98. Indirect evaluation of collateral vessels can not only detect their presence but also establish their hemodynamic significance with high accuracy.