UNLABELLED Following parenteral administration, clonidine has analgesic effects at both cerebral and spinal levels. Patient-controlled analgesia (PCA) makes it possible to determine equipotent dosages of analgesics by relating analgesic consumption per time to the levels of analgesia obtained in comparable patient populations. Therefore, we studied the equipotency ratios of clonidine and piritramide and the incidence of undesired side effects in the treatment of postoperative pain in patients undergoing maxillo-facial surgery. METHODS After approval of the local ethics committee and informed consent 40 patients (age > 18 year, ASA I-III) were studied. Following randomization, the patients each received a PCA device containing either clonidine (bolus 30 micrograms), or piritramide (bolus 1.5 mg) for treatment of postoperative pain (lockout interval 5 min in both groups). During the postoperative period, pain was determined using a visual analogue scale, while analgesic consumption, sedation, haemodynamic parameters, respiration rate, and the occurrence of undesired side effects were documented additionally. RESULTS The groups had comparable distributions of biometric data, duration of anaesthesia, and ASA classification. Pain level decreased significantly (P < 0.0001) in both groups during the first 2 h of PCA. Mean arterial pressure and heart rate were lower (P < 0.05) in the clonidine group 4 and 6 h after PCA onset, while the degree of sedation after 2 (P < 0.01) and 6 (P < 0.05) h was higher than in the piritramide group. Nausea and vomiting were more frequent (P < 0.05) in the piritramide group. Both groups showed a wide interpatient variation in analgesic requirement. The equipotency ratio clonidine/piritramid was 1:63.7. CONCLUSION Intravenous clonidine is a potent analgesic and is suitable or the treatment of postoperative pain following maxillo-facial surgery. The analgesic potency of 150 micrograms clonidine i.v. was equivalent to that of 9.56 mg piritramide i.v. Nausea and vomiting occurred more rarely in the clonidine group, while deeper sedation was observed more frequently than in the piritramide group. Owing to the wide interindividual variation of analgesic consumption, clonidine dosages have to be adjusted to the actual requirements.