Modern ultrasonic Doppler systems enable us for the first time to carry out intraoperative and pre- and postoperative investigations of the circle of Willis. The application of Doppler in acute aneurysm surgery provided the following information: During the first 3 days following subarachnoid bleeding from a cerebral aneurysm, no signs of vasospastic reaction were found by Doppler sonography. Thereafter, 90% of all patients operated on in the acute stage demonstrated localized acceleration as a sign of vasospasm, despite prophylactic nimodipine treatment. Thirty-eight percent of the patients developed delayed ischemic deficits, which, however, were only transient. These results may indicate that early surgery and nimodipine prophylaxis may not prevent vasospasm but do reduce its consequences.