Idiopathic lumbosacral plexopathy is an uncommon idiopathic disorder characterized by acute onset of severe lower extremity pain, followed by weakness, atrophy of affected muscle, and variable sensory disturbance. A 61-year-old man experienced sudden onset of dysesthesia with weakness in right lower extremity. Electrodiagnostic study revealed patch pattern denervation at L3 and L4 root without paraspinal muscle involvement. Lumbar MRI showed abnormal signals in the right lumbar plexus. Pulse intravenous administration of high-dose immunoglobulin alleviated the sensorimotor symptoms. Abnormal signals in lumbar MRI reduced 6 months later. (J Korean Acad Rehab Med 2009; 33: 723-727)