The dose requirements and side effects of morphine were compared with those of diamorphine administered by patient-controlled analgesia in 40 patients following elective total hip replacement. Patients were allocated randomly to receive in a double-blind manner either morphine or diamorphine for postoperative pain relief. There were no significant differences between the two groups with regard to postoperative sedation, nausea, well-being, pain relief and requirements for antiemetic drugs. The dose requirement for diamorphine was approximately 50% of that for morphine.