Patients may become sedated with spinal anesthesia; however, the effect of the extent of spinal block on the Bispectral index (BIS), a processed electroencephalographic variable, has not been fully investigated. We evaluated the influence of the extent of spinal block on BIS values and on regional cerebral oxygen saturation (rSO(2)) in elderly patients. A prospective, randomized, double-blinded study was performed in 55 ASA II patients undergoing urological surgery. The patients were randomly allocated into one of two groups to receive 2.7 ml of 0.5% hyperbaric bupivacaine or 1.5 ml, and then divided into two groups according to level of spinal blockade: high spinal group (Th6 and above) or low spinal group (Th12 and below). Systolic blood pressure (SBP), heart rate (HR), cardiac output (CO), stroke volume (SV), BIS values, and rSO(2) were measured for 30 min. CO and SV were evaluated using impedance cardiograph methods. The level of spinal blockade was Th4.7 +/- 1.0 in high spinal group (n = 20) and L2.5 +/- 2.2 in low spinal group (n = 20). High spinal anesthesia produced a significant decrease in SBP (p < 0.01) and SV (p < 0.01), but had no effect on CO. High spinal anesthesia significantly decreased BIS values (p < 0.01) without affecting rSO(2). There was relationship between level of spinal blockade and BIS values (r = 0.566). In contrast, no changes in above parameters were found in low spinal group. This study provides evidence that the extent of spinal block may have significant influence on BIS values without affecting rSO(2) in elderly patients. Nishikawa, Hagiwara, Nakamura, Ishizeki, Kubo, Saito, Goto, , (2007). The effects of the extent of spinal block on the BIS score and regional cerebral oxygen saturation in elderly patients: A prospective, randomized, and double-blinded study. Journal of clinical monitoring and computing, 2007 Apr;21(2):109-14. https://www.ncbi.nlm.nih.gov/pubmed/17216322