BACKGROUND: Opioids can produce potent antinociceptive effects by interacting with local opioid receptors in inflamed peripheral tissue. However, reports on pain relief with intra-articular morphine after arthroscopic knee operations are conflicting. In this study we examined the analgesic effects of the intraarticular administration of morphine after knee surgery. METHODS: In a double-blind, randomized trial, we studied 26 patients who had received one of two injections at the end of surgery. The patients in group M(n=11) received 3 mg of morphine intraarticularly; those in group P(n=15), saline 20 ml intraarticularly as a placebo. RESULTS: Patients in the morphine group had significantly lower pain scores throughout the 24-h postoperative period compared with those in the placebo group(P<0.05). There was less requirement for supplementary analgesics in the morphine group. CONCLUSIONS: Low doses of intraarticular morphine can significantly reduce pain after knee surgery without any systemic side effect.