Cilostazol (6-[4-(1-cyclohexyl-1H-tetrazol-5-yl)butoxy]-3,4-dihydro-2 (1H)-quinolinone, OPC-13013) is an anti-platelet aggregating and vasodilating drug. But cilostazol is known to have a tendency to cause headache, which is thought to be due to the dilatation of the external carotid artery. In the present study the effect of cilostazol on the blood flow in the carotid arteries was established by the Doppler ultrasound technique. Twelve patients with cerebral infarction (mean age 66.3 years) were divided into two groups consisting of 9 patients (Group 1) who did not have headache during treatment with cilostazol and 3 patients (Group II) who complained of headache. The systolic peak frequency (PEAK) of the common and external carotid arteries was measured using a Doppler ultrasound device. The PEAK of the common and external carotid arteries were examined before and 4 weeks after oral administration of 200 mg/d, 100 mg twice a day, cilostazol. In Group I, the PEAK in bilateral common carotid arteries increased significantly after treatment with cilostazol, but the PEAK in bilateral external carotid arteries showed no significant change. In Group II, the PEAK in both common carotid arteries showed no significant change after treatment with cilostazol, but the PEAK in the right external carotid artery increased significantly.