INTRODUCTION: In our study, we aimed to compare the analgesic efficacy, side effect profile and its effect on opioid consumption of the femoral nerve block applied with different concentrations of local anesthetic agents (%0.5 bupivacaine and %0.25 bupivacaine) in the same volume (20 mL) after total knee replacement (TKR) operation. METHODS: The files of patients who underwent unilateral TKR surgery under spinal anesthesia between August 2018 and June 2019 were retrospectively reviewed. A total of 163 patients were included in the study, 81 patients in group 1 who received %0.25 bupivacaine, and 82 patients in group 2 who received %0.50 bupivacaine for femoral block. The highest visual analogue pain scale (VAS) scores in the postoperative 24 hours, the amount of tramadol requested and consumed with intravenous patient-controlled analgesia (PCA) devices, and whether there was a significant difference in terms of side effects (nausea, vomiting, motor and sensory deficit) were analyzed. RESULTS: The highest VAS score in group 1 was 2.95 ± 1.31, in group 2 it was 2.84 ± 1.06, and there was no significant difference between them. The mean consumption of tramadol was 197.04 ± 92.03 mg in group 1 and 208.05 ± 85.06 mg in group 2. There was no difference between the demand and consumption of tramadol and side effects. DISCUSSION AND CONCLUSION: 20 mL %0.25 bupivacaine for the femoral block provided the equivalent analgesic efficacy to the same volume of %0.50 bupivacaine. We think that the use of % 0.25 bupivacaine is a more reliable option to reduce systemic side effects, motor block risk and complications.