Anterior choroidal artery (AChA) syndrome is still one of the most serious complications of the clipping of internal carotid artery aneurysms. No monitoring method can detect ischemia in the area of the AChA during surgery. This artery may be obstructed when a clip is applied to the neck of the aneurysm, and patency is sometimes difficult to confirm by microscopy because of the artery's small size and site of origin (usually behind the internal carotid artery as viewed surgically). However, microvascular Doppler sonography (MVDS) can detect flow instantaneously even in such a small vessel. In our series, AChA syndrome occurred in three of 19 patients treated for AChA aneurysm before the introduction of MVDS, but only one of 19 patients treated with the aid of this device. In that patient, one of the two AChA branches was intentionally sacrificed by applying a clip to the prematurely ruptured aneurysm. MVDS detected hypoperfusion of the AChA after clipping in five other patients, and so the clip was readjusted to preserve AChA flow. Use of MVDS is very effective to prevent inadvertent injury to the AChA during aneurysm surgery on this artery.