BACKGROUND: The addition of bupivacaine to lipid soluble opioids have improved analgesia and can reduce opioid related side effects in the epidural analgesia. The use of the combination of bupivacaine and lipid soluble opioid fentanyl is gaining in popularity after surgery. But recent work has produced conflicting results, and the ideal combination of local anesthetics and opioid for Patient Controlled Epidural Analgesia (PCEA) is unknown. We have compared the efficacy and the safety of 0.1% bupivacaine and 0.2% bupivacaine combined with fentanyl 5 microgram/ml for PCEA. METHODS: Forty women undergoing lower abdominal gynecological surgery under general anesthesia were assigned to receive an epidural initial bolus of 0.1% (0.1% group) or 0.2% bupivacaine (0.2% group) combined with 5 microgram/ml fentanyl mixture 10 ml, followed by a PCEA with 2 ml/hr continuous background infusion and demand-dose of 2 ml in prospective, random, double blind fashion. Resting and coughing visual analogue scale (VAS) for pain, infusion rates of the mixture, and side effects were recorded at 4, 8, 24, 48 hr postoperatively, and satisfaction were noted after 48 hr. RESULTS: No differences in resting and coughing VAS scores, hypotension, nausea, itching sensation, and somnolence were observed between the two groups. The mean infusion rates of mixtures during the first 4 hr were higher in the 0.1% group (3.3 1.3 ml/hr) than the 0.2% group (2.5 0.6 ml/hr) (p<0.05). The incidence and the degree of sensory and motor block were higher in the 0.2% group (p<0.05). CONCLUSIONS: Both groups provided sufficient postoperative analgesia and satisfaction with some side effects. The high incidence and degree of sensory and motor block were resulted in the 0.2% group, which is not optimal for early ambulation. To search for more titrated bupivacaine concentration between 0.1% and 0.2%, loading doses or appropriate epidural puncture level having reduced demand dose and decreased side effects, further studies are expected.