50 patients scheduled for total knee arthroplasty under spinal anaesthesia were randomized to either intrathecal morphine or intrathecal fentanyl to be added to bupivacaine for spinal anaesthesia. Regarding postoperative pain, VAS pain scores were significantly lower throughout follow-up from 2-24 hours in the morphine group compared to the fentanyl group, and the morphine group demonstrated a significantly longer time to first rescue analgesia request when compared to the fentanyl group. No significant differences between groups were observed in the incidence of PONV.