In diabetic patients, orthostatic hypotension is often found as one expression ofautonomic dysfunction. We therefore studied the impairment of the autoregulation of the cerebral circulation in diabetics with orthostatic hypotension. The subjects consisted of 20 controls and 7 patients with diabetic orthostatic hypotension. Rheoencephalography was used to estimate the cerebrovascular resistance (CVR). That is to say, we measured the anacrotic section of the rheoencephalograrn from its initial rising point to its top, and then calculated its relative part towards the single cardiac cycle. This parameter provides information on vascular tone and cerebral resistance. The cerebral blood flow (CBF) was calculated by dividing the mean arterial blood pressure (MABP) by the CVR. The changes in CVR and CBF between the supine and tilting positions were analyzed quantitatively by using the following dysautoregulation indices (DAI): DAICVR=-Change in CVR (%)/Change in MABP (mmHg) DAICBF=-Change in CBF (%)/Change in MABP (mmHg) The DAICVR was +0.60±0.73 in diabetics and -0.63±0.73 in controls, suggesting a significant increase in CVR in diabetics (p<0.01). On the other hand, the DAICBF was-1.34±0.32 in diabetics and-0.40±0.75 in controls, suggesting a significant decrease in CBF in diabetics (p<0.01). The above results indicate that the autoregulation of the cerebral circulation may be impaired in diabetic orthostatic hypotension.