BACKGROUND: Differences between sexes in sensitivity to nociceptive stimuli and response to analgesics have been noted in humans and animals. Male rats are more sensitive than female rats to the antinociceptive properties of morphine. Kappa-opioid analgesia is greater in women than in men. The purpose of this study was to evaluate the difference between the sexes in postoperative analgesic responses to patient-controlled epidural analgesia (PCEA) with butorphanol or fentanyl in 0.05% bupivacaine solution. METHODS: After obtaining their consents, 30 men and 30 women, less than 60 years old, undergoing elective gastrectomy under general anesthesia were randomly allocated into four groups of 15; butorphanol-female, butorphanol-male, fentanyl-female and fentanyl-male. An epidural catheter was introduced at the T7-8 or T8-9 interspinous space before the operation. Postoperative analgesia was provided with PCEA with butorphanol 50 microgram/ml or fentanyl 5 microgram/ml in a 0.05% bupivacaine solution. When patients first required analgesics after complete recovery of consciousness from anesthesia, a bolus of 5 ml was initially administered. The PCEA device was set to deliver a bolus of 2 ml, a lockout interval of 10 min and no basal infusion. PCEA consumption, pain intensity using a 10-cm visual analog score (VAS), patient's satisfaction to PCEA and side effects were evaluated at 3, 6, 24, and 48 h after the surgery. RESULTS: Cumulative PCEA consumption with butorphanol was less in the butorphanol-female group than in the butorphanol-male group at 24 and 48 h postoperatively. VAS pain scores and patients' satisfaction to PCEA were not different between the butorphanol-female group and the butorphanol-male group. Cumulative PCEA consumption with fentanyl was less in the fentanyl-male group than in the fentanyl-female group at 6, 24 and 48 h postoperatively. VAS pain scores and patients' satisfaction to PCEA were not different between the fentanyl-female group and the fentanyl-male group. There was no differece in side effects in both butorphanol groups and in both fentanyl groups. CONCLUSION: For postoperative analgesia with PCEA, butorphanol may be better for females than males, and fentanyl may be better in males than in females.