Objective To evaluate the effect of dexmedetomidine on cognitive dysfunction after thoracic surgery in patients undergoing one-lung ventilation.Methods Sixty-two patients,aged 45-64 yr,of ASA physical status Ⅰ or Ⅱ,with body mass index ranged from 17.5 to 25.5 kg/m2,scheduled for elective thoracic surgery,were randomly allocated into 2 groups (n =31 each) using a random number table:dexmedetomidine group (Dex group) and control group (C group).Dexmedetomidine 0.5 μg/kg was infused for 10 min starting from the time point before induction of anesthesia,followed by continuous infusion at a rate of 0.5 μg · kg-1 · h-1 until 30 min before the end of surgery in Dex group.The equal volume of normal saline was administered instead in group C.Before induction and at 24,48 and 72 h after surgery,venous blood samples were collected for determination of levels of S-100 beta protein and neuronspecific enolase in serum by ELISA.Cognitive function was assessed by Mini-Mental State Examination at 72 h after surgery.Results The levels of S-100 beta protein and neuron-specific enolase in serum were significantly increased after surgery than before induction in the two groups.Compared to C group,the levels of S-100 beta protein and neuron-specific enolase in serum were significantly decreased after surgery,and the incidence of postoperative cognitive dysfunction was decreased in Dex group (26％ vs 6％).Conclusion Dexmedetomidine can effectively reduce the nerve damage during one-lung ventilation and significantly inhibit the development of postoperative cognitive dysfunction in patients undergoing thoracic surgery,indicating that dexmedetomidine is suitable for thoracic surgery.