Transcranial Doppler (TCD) was utilized to estimate the cerebral blood flow velocity during postural changes or walking in two patients with idiopathic orthostatic hypotension (IOH). The probe of TCD was attached tightly to the temporal skull with a headband to measure the flow velocity of middle cerebral artery (MCA). The patients' postures were altered passively by a tilting table or actively by standing up or walking with their own feet. The TCD study clearly demonstrated a complete loss of autoregulation in Case 2 and an incomplete impairment of autoregulation in Case 1. In both patients, the blood pressure dropped markedly at a standing position, and MCA flow velocity decreased accordingly resulting in manifestation of dizziness. In Case 1, the blood pressure was elevated during walking, and MCA flow velocity increased leading to the improvement of ischemic symptoms. In Case 2, the TCD study was repeated after administration of fluorohydrocortisone, a mineralocorticoid agent, which ameliorated clinical symptoms. The results, however, indicated no improvement of autoregulatory response. The amelioration of symptoms was attributable to the elevation of blood pressure levels. Our study indicates the capability of TCD to estimate cerebral blood flow alteration during postural changes or walking. The TCD appears to be useful for evaluating the autoregulatory response and effectiveness of therapy in patients with orthostatism, like IOH.