Clonidine has been reported to produce analgesia in humans in different painful conditions. The aim of the present study was to investigate if epidural clonidine produced a clinically important analgesia to severe postoperative pain. Using a controlled, randomized double-blind design, one group of patients received a single dose of epidural clonidine 3 micrograms/kg (n = 10) and a control group epidural 0.9% saline (n = 10), when reporting postoperative pain after thoracotomy performed under standardized anaesthesia. To quantify the effects of the given epidural drugs, the need for supplementary, intravenous pethidine analgesia was recorded. The patients had access to the supplementary analgesic by means of a patient-controlled analgesic-delivery device. The two groups were similar regarding anthropometric and clinical data. Epidural clonidine 3 micrograms/kg did not affect the need for supplementary intravenous pethidine analgesia, as compared to the control group at any time during the first 12 h postoperatively. The side-effects of epidural clonidine were tolerable, and no treatment for arterial hypotension was required. No early or delayed respiratory depression occurred. In conclusion, clonidine 3 micrograms/kg epidurally seems to lack clinically important analgesic effects on severe postoperative pain, at least following thoracotomy.