Flow disturbances in main cerebral arteries may cause severe neurological symptoms. Using transcranial Doppler sonography (TCD) the blood flow velocities in the basal cerebral arteries (BCA) can be recorded at any age. Transient stenoses or occlusions of main cerebral arteries were detected in 11 children by this method and confirmed by other techniques. Vasospasm produced a marked increase in flow velocities in the affected arteries which was reduced by nimodipine, the calcium channel blocker. Vasospasm also occurred in severe bacterial meningitis. In acute hemiplegia due to cerebral arterial obstruction no flow velocities could be recorded at the corresponding site. If distal branches were obstructed reduced flow velocities were found proximally. Increased flow velocities or reversed flow in anastomoses indicated the collateralization. The transient nature of the occlusions was shown by repeated recordings. TCD is a reliable, noninvasive and rapidly available technique for diagnosing or excluding transient flow disturbances in the main cerebral arteries as the cause of neurological symptoms in children. It indicates the necessity and most advantageous stage for therapy.