Background: Contraction of anal sphincter muscle produces severe pain in perianal surgery postoperatively. Recent reports have indicated that effective and prolonged pain relief can be obtained by the injection of small doses of morphine into the subarachnoid space. We attempted to use this technique for perianal surgery and investigated postoperative pain control and its side effects. Methods: Forty five patients scheduled for hemorrhoidectomy and anal fistulectomy were studied to determine the minimal effective dose of intrathecal morphine for postoperative analgesia. In order to control the pain, 7 mg of 0.5% hyperbaric bupivacaine with 0.05 mg (group I), 0.1 mg (group II) and 0.15 mg (group III) of morphine hydrochloride was injected with a 25 gauge spinal needle into the subarachnoid space. We estimated the duration of analgesia until the pain score attained to above 3 in 10 cm VAS (visual analogue scale) and incidence of itching, nausea and vomiting by percentage, headache, backpain and respiratory depression by positive and negative. We also checked the time of self-voiding. Results: The mean time of analgesia was $10.3{pm}1.54$, $19.7{pm}2.22$ and $20.3{pm}2.29$ hours in group I, II and III respectively. Urinary retention of group I, II and III after block persisted for an average of $20.3{pm}2.31$, $21.2{pm}2.51$ and $23.3{pm}3.74$ hours. Nausea and vomiting were observed 33%, 53%, 67% and itching was observed 53%, 67%, 80% in group I, II and III respectively and respiratory depression did not occur in all groups. Conclusions: It is not necessary to use more than 0.1mg of intrathecal morphine in perianal surgery because analgesia is not prolonged and side effects are increased.