PURPOSE: The aim of the study was to compare the analgesic effect of 5 mg intra-articular (IA) morphine with 50 mg IA tramadol. TYPE OF STUDY: Prospective double-blind randomized trial. METHODS: Seventy-five patients having elective arthroscopic surgery of the knee were randomized to receive IA tramadol 50 mg (tramadol group), IA morphine 5 mg (morphine group), or IA normal saline (control group), in equivalent volumes (20 mL). The tourniquet was released 10 minutes after analgesic administration. Verbal pain rating score between 0 and 10 (VRS), supplemental analgesic requirements, and incidence of side effects were recorded postoperatively. RESULTS: Results are given as (median [5-95 percentiles]). The control group had a significantly shorter time to first analgesic request (25 min [15-55]) than morphine group, (34 min [15-158], P < .008) and the tramadol group, (33 min [17-728], P < .008). The patients in the control group complained of more severe pain (VRS 7 [4-10]) when they arrived at the postanesthesia care unit compared with the morphine group (VRS 1 [0-9], P = .002) and with the tramadol group (VRS 0 [0-9], P = .002). These treatment benefits were especially prominent in the patients who had meniscectomy or in the subgroup of patients with more than 6 months of preoperative pain. There was no statistical difference between the tramadol and morphine groups in the time to first analgesia, postoperative pain scores after arrival at the postanesthesia care unit, consumption of rescue analgesic, or side effects. CONCLUSIONS: We conclude that 50 mg IA tramadol provides analgesia equivalent to 5 mg IA morphine. LEVEL OF EVIDENCE: Level II, randomized controlled trial that shows no significant difference and lacks narrow confidence intervals.