Background: In December 2001 we used 0.2mg intrathecal morphine at the same dose as in post cesarean analgesia for transurethral resection of prostate (TUR-P) cases but this technique was not effective. This study assessed the analgesic effectiveness of an intrathecal morphine dose in the rang from 0.5 to 1.5 mg. Methods: A total of 45 ASA I or II patients scheduled for TUR-P surgery were randomly assigned to three groups. Group A patients served as reference and were anesthetized by intrathecal injection of 0.5% bupivacaine (20mg／4mL／amp) 10 mg and 0.5 mg morphine 10mg／mLamp). Group B and C patients underwent the same anesthetic procedure but morphine was injected simultaneously at a dose of either 1mg (Group B) or 1.5 mg (Group C). Results: Group A patients experienced postoperative pain for 4 to 6 hours and required a mean intravenous dose of 50 mg of meperidine. Group B and C patients had a satisfactory degree of analgesia. Nevertheless, there was no significant difference in the analgesia scores between groups C and B. None of the patients in these groups developed respiratory impairment, but Group C had a higher number of patients with delayed vomiting than the other groups. Conclusion: This study found that 1 mg of intrathecal morphine appeared to be the optimal dose for analgesia after TUR-P.