This study compared the analgesic efficacy of an epidural infusion of ropivacaine and ropivacaine with sufentanil following major knee surgery. In a double-blind clinical trial, 115 adult patients received either epidural ropivacaine (R group, 2 mg.ml(-1)), or ropivacaine (2 mg.ml(-1)) with sufentanil (RS group, 1 microg.ml(-1)), using a patient-controlled epidural analgesia technique. Pain scores (visual analogue scale, VAS, and the simple descriptive scale, SDS), side-effects, motor block and treatment quality were recorded at 6, 12 and 24 h after the insertion of the epidural catheter. In the RS group, analgesic efficacy was significantly greater than in the R group between 12 and 24 h following insertion of the epidural catheter (VAS: 92.9% vs. 72.9%, p = 0.009). There was no significant difference during the other periods. Pruritus, nausea and vomiting were significantly more frequent in the RS group. Good postoperative analgesia was obtained with an epidural infusion of ropivacaine (2 mg.ml(-1)). When this local anaesthetic was administered with sufentanil, there was an improvement in the analgesic effect but a significant increase in the number of patients who reported adverse effects. The differences were more pronounced 12 h after the beginning of the analgesic schedule. This study failed to demonstrate any worthwhile clinical benefit from the addition of sufentanil.