We hypothesize that depth of anesthesia may exert an influence on the early postoperative cognitive dysfunction. One reason that deep anesthesia might protect cognitive function is the prevention of the stress response to noxious stimulation. The noxious stimulation of surgery can induce stress responses whose severity correlates with the intensity of surgery (major versus minor).Light anesthesia with a serious stress response to noxious stimulation may increase the incidence of POCD. Furthermore, major surgery causing a serious stress response can give rise to a higher incidence of early postoperative cognitive dysfunction than minor. This may also prove our hypothesis on the other hand. The elderly patients who received major surgery had a higher incidence of early POCD than those who received minor surgery. A deleterious stress response can be associated with major surgery or light anesthesia, and it can cause excessive production of corticosteroids that target receptors in the cerebral cortex and subcortical nucleus, eventually resulting in neuron injury. This may explain why the elderly has a higher incidence of early POCD after surgery. However, the effect of the depth of anesthesia on POCD remains controversial. Farag E et al. randomized 74 patients to either a low Bispectral Index (BIS) regimen or a high BIS regimen during the surgical procedure. Preoperatively and 4-6 week after surgery, the patients' cognitive status was assessed with a cognitive test battery consisting of processing speed index, working memory index, and verbal memory index. Processing speed index was higher in the low BIS group versus the high BIS group. But no difference was observed in the other two test battery components. Certainly one of the limitations of their data is that the observed difference in cognitive ability occurred in only one of three assessments. Recently, we have made a randomised, double-blind, controlled study to demonstrate that deeper general anesthesia, as defined by a median BIS of 38, when compared with a median BIS of 58, was associated with a reduced incidence of early POCD(at 1 week postoperationly). This study has been published in J Neurosurg Anesthesiol , Volume 23, Number 1, January 2011. as it was just a small sample(n=40) study without considering incidence of long-term POCD, we hope to obtain more reliable data from a multi-center study to investigate whether the depth of total intravenous anesthesia using propofol affects the early and long-term incidence of POCD.