Sixty patients undergoing elective total hip replacement under spinal anaesthesia were randomly assigned to receive either intrathecal (IT) diamorphine 0.75 mg (n = 30) or IT morphine 1.0 mg (n = 30). Postoperative pain scores, analgesic requirements and side effects were assessed by a blinded observer. Postoperative pain scores were broadly similar and satisfactory for both groups but the amount of additional IV morphine required to achieve this was significantly reduced in the morphine compared with the diamorphine group (P < 0.05). Twelve of the morphine group required no postoperative analgesics compared with four in the diamorphine group (P < 0.02). There were no differences between the groups in the incidence of side effects such as emesis and pruritus. No significant postoperative respiratory depression was noted. In the doses used intrathecal morphine provided superior postoperative analgesia to that of intrathecal diamorphine.