Background and objective: Oral morphine may be useful for postoperative pain relief, but few studies have tested its use after in-hospital surgery. Methods: We evaluated clinical efficacy and the pharmacokinetic parameters of oral morphine after total hip arthroplasty. We recruited 60 patients who had total hip arthroplasty under general anaesthesia. The patients were randomized to receive placebo, 10 mg morphine sulphate or 20 mg morphine sulphate orally every 4 h for 24 h. The oral administration was started 3 h after the morphine-loading dose in the Post Anaesthesia Care Unit and then patients used intravenous morphine patient-controlled analgesia for 24 h. Pain score at rest (scored by patients on a visual analogue scale), sedation, nausea, vomiting and urinary retention were monitored. In 11 additional total hip arthroplasty patients, we determined the pharmacokinetics of morphine and its metabolites after oral administration of 20 mg morphine sulphate every 4 h for 16 h. Results: The amount of morphine administered via patient-controlled analgesia over 24 h was reduced in the 20-mg group compared with that in the placebo group (19.0 ± 2.7 mg vs. 33.0 ± 5.5 mg; P = 0.03). No significant morphine-sparing effect was observed in the 10-mg group. Pain scores and side-effects were similar in all groups. The pharmacokinetic study revealed a limited and slow absorption of morphine. Conclusion: Despite a limited absorption of oral morphine postoperatively, high doses of oral morphine have a significant analgesic effect after total hip arthroplasty.