One hundred patients requiring appedectomy were studied to determine the minimal effective dose of intrathecal morphine for postoperative analgesia. In double-blind fashion, groups of 20 patients received either 0.02 mg (group I), 0.04 mg (group II), 0.06 mg (group III), 0.08 mg (group IV), or 0.10 mg (group V) intrathecally with 10% dextrose in water 2 ml. Group II to group V patients reported significantly less postoperative pain than group I patients as assessed by the Prince Henry pain scale and required significantly fewer analgesic interventions for 24 hours. The incidences of vomiting and pruritus were considerably high in all groups, but none of them required any treatment. The incidence of urinary catheterization due to urinary retention in group II to V was twice that of group I. No clinically evident respiratory depression occurred in any of the subjects. In conclusion, intrathecal morphine administration of 0.04 mg proved effective in reducing postoperative analgesic requirements and in eliminating postoperative pain following appendectomy and was not associated with significant side effects. It is very likely that such low dose intrathecal morphine would also work in other operations.