Epidural injection of fentanyl added to 0.5% bupivacaine improves epidural anaesthesia during caesarean section. The present prospective randomized double-blind study sought to determine the lowest effective dose of fentanyl. Eighty healthy women at term were divided into four groups of 20, with each group receiving a different 2 ml study solution: saline (control) or 50, 75 or 100 microg of fentanyl added to 20 ml of 0.5% bupivacaine. Additional increments of 0.5% bupivacaine were administered as required at 30 min and onwards until a bilateral block to T4 was attained. The onset, duration and segmental level of analgesia, and degree and duration of motor block were not influenced by the addition of fentanyl. Peroperative quality of analgesia was improved, as the request for supplementary analgesics was less following the addition of 75 or 100 microg of fentanyl (P < 0.05). Postoperatively the time to first supplemental opioid request was prolonged and pain was reduced for at least 6 h from induction of anaesthesia when 75 or 100 microg of fentanyl had been added (P < 0.05). Pruritus was the only side-effect (P < 0.05). In conclusion, fentanyl 75 microg was the lowest effective dose for improving quality of analgesia. Onset time was not reduced by the addition of fentanyl.