Three patients with chronic paroxysmal hemicrania (CPH) (1 M, 2 F) and 9 healthy controls (8 M I F) were studied with transcranial Doppler (TCD) sonography. One patient who was studied during the spontaneous attacks hyperventilated markedly. Middle cerebral artery velocity (VMCA) was measured in the first attack, and anterior cerebral artery velocity (VACA) in the second attack, respectively. VMCA and VACA decreased bi-laterally during attack. VMCA started to decrease at an early stage of the attack, i.e. prior to the major hyperventilation that was observed during the attack. VACA on the symptomatic side decreased less than that on the other side (P<0.05). Cerebral vasomotor reactivity (VMR) was expressed as the percentage change in mean blood flow velocity as a function of end-tidal PCO2 (PETCO2) reduction induced by voluntary hyperventilation (DV/DPETCO2). In the 3 patients, a slightly lowered VMR was observed in the MCA and posterior cerebral artery on both sides, and in the ACA on the symptomatic side IP>0.05) in comparison with controls. These observations may imply an abnormal vascular reactivity in CPH.