Intrathecal (IT) morphine provides excellent postoperative analgesia but may result in many side effects, including postoperative nausea and vomiting, pruritus, and respiratory depression, particularly at larger doses. Older patients may be at particular risk. The optimal dose of spinal morphine in older patients undergoing hip arthroplasty is not known. We designed this prospective, randomized, controlled, double-blinded study to evaluate the analgesic efficacy and side effect profile of 50-200 mug of IT morphine in older patients undergoing elective hip arthroplasty. Sixty patients older than 65 years undergoing elective hip arthroplasty were enrolled. Patients were randomized to receive spinal anesthesia with 15 mg of bupivacaine and IT morphine in four groups: 1) 0 mug, 2) 50 mug, 3) 100 mug, and 4) 200 mug. IT morphine 100 and 200 mug produced effective pain relief and decreased the postoperative requirement for morphine compared with control. IT morphine 50 mug did not provide effective pain relief. Both 100 and 200 mug of IT morphine provided comparable levels of postoperative analgesia. There were no between-group differences in postoperative nausea and vomiting, sedation, respiratory depression, or urinary retention. Pruritus was significantly more frequent with 200 mug of IT morphine. In conclusion, 100 jig of IT morphine provided the best balance between analgesic efficacy and side effect profile in older patients undergoing hip arthroplasty.