A 55-year-old woman received a combined spinal epidural anesthesia for total replacement surgery of the left knee. After surgery, the epidural patient controlled analgesia (PCA) device was activated. After the first post-operative day, the patient complained of severe back pain rather than left knee pain in spite of the bolus dose of analgesics via epidural catheter. The epidural catheter was removed and intravenous PCA was initiated. On the 16th post-operative day, the patient's back and knee pain improved; however, the fever did not subside. A blood test revealed increased inflammatory markers in spite of antibiotic therapy. Under the suspicion of epidural abscess, a lumbar magnetic resonance imaging scan was performed and revealed an increase in the signal within left lumbar paraspinal muscle. A prompt debridement and irrigation of the necrotic tissues was performed. The histologic diagnosis was lumbar paraspinal myonecrosis. Subsequent to this corrective procedure, the patient's fever subsided and the inflammatory markers were normalized, except for mild back pain after the surgical repair.