Objective: To deduce optimal fentanyl infusion dose for effective analgesia with minimal side effects and maximum hemodynamic stability. Material and methods: In our prospective study, comparing the three groups (of 30 patients each) namely group 2, 3, 4 receiving three different doses of fentanyl (20 µg, 30 µg, 40 µg), respectively along with control group (Group 1) receiving conventional analgesics through intramuscular or intravenous route. Effective analgesia rated on linear visual analog scale (VAS) with minimum side effects and most stable hemodynamic parameters. Results: The VAS scores, at rest, were significantly lower for epidural fentanyl groups as compared to control group. Mean blood pressure and pulse rate in all groups were comparable at all times. The incidence of side effects was similar in the three groups as compared to control group. Conclusion: Fentanyl dose of 40 µg is thus the optimal epidural dose of background infusion along with patient on demand analgesia in terms of maximum analgesic efficacy, maximum hemodynamic stability and minimum side effects in patients undergoing unilateral total knee replacement.