Objective To investigate the ED50 and ED9 5 of subarachnoid 0.375% bupivacaine in elderly patients undergoing elective total hip arthroplasty (THA ) in high altitude area. Methods Twenty-three elderly patients,aged 73-99 years old,ASA Ⅱ or Ⅲ,selected for total hip ar-throplasty (THA)under combined spinal-epidural (CSE)anesthesia were enrolled in this study and predose of 3.0 mg,0.375% (m/v)bupivacaine administered in subarachnoid,following dosage in-creased by ratio of 0.8.Calculate the ED50 and ED9 5 of subarachnoid 0.375% bupivacaine and 95%CI.Results Twenty of 23 underwent the test completely.The ED50 (95%CI)and ED9 5 (95%CI)of 0.375% (m/v)bupivacaine were 4.81 mg (3.98-5.81 )and 7.29 mg (6.03-8.81 ),respectively. Conclusion Subarachnoid 0.375% bupivacaine (7.5 mg)can be used effectively and safely in elderly patients undergoing total hip arthroplasty in high altitudes area.