We have studied the efficacy of a continuous i.v. infusion of diclofenac 2 mg kg−1/24 h given for 2 days after major thoracic surgery in 30 patients in a double-blind, placebo-controlled, parallel-group design. The patients were able to obtain additional pain relief as on demand morphine blouses. In the diclofenac group, the consumption of morphine was reduced by 60% during the first and by 76% during the second day after operation compared with the control group. Overall, analgesia was also superior in the diclofenac group. Arterial oxygen-ation was significantly greater and the arterial CO2 increased less during the first day after operation in the diclofenac group compared with the control group. Diclofenac had no significant effect compared with placebo on blood loss or on any bleeding or platelet test. Urine output was significantly less during the first day after operation in the diclofenac group compared with the control group, but was normal on the second day after operation; plasma creatinine concentrations were unchanged. I.v. diclofenac infusion combined with opioids delivered via a patient-controlled analgesia device seems a valuable method of pain relief after thoracic surgery in patients in whom more invasive techniques, such as extradural local anaesthetics and opioids, cannot be used. However, non-steroidal anti-inflammatory drugs should be used cautiously, if at all, in patients who are at risk of acute renal failure.