Tramadol, a weak opioid mu-receptor agonist, may have a favourable potency and side effect profile for intravenous patient-controlled analgesia (PCA). In a prospective, double-blind, randomized study involving 54 patients, tramadol was compared with oxycodone in PCA after maxillofacial surgery. All the patients were given diclofenac sodium 1 mg kg-1 intramuscularly and dexamethasone 8 mg twice a day. Post-operatively patients received tramadol or oxycodone by a PCA apparatus (lockout 5 min, tramadol 0.3 mg kg-1 bolus, oxycodone 0.03 mg kg-1 bolus). During the immediate recovery period, opioid was administered i.v. in a double-blind fashion, either tramadol 10 mg or oxycodone 1 mg increments until the pain control was judged to be satisfactory by the patient. Pain was assessed at rest and during activity (mouth opening) before and after loading, at 2 h after commencing the PCA, as well as at 21.00 and at 09.00 hours on the following morning. Side effects were recorded. The potency ratio of tramadol to oxycodone was found to be approximately 8:1. There was no significant difference between the groups in the VAS scores for pain. No respiratory depression was identified. Tramadol was found to provide adequate analgesia after maxillofacial surgery without risk of respiratory depression. However, the incidence of nausea was slightly greater in the tramadol group than in the oxycodone group (44% vs. 28%, NS).