The diagnosis of peripheral neuropathy had been performed by electrophysiologic studies and neurologic examination. However, Magnetic Resonance Imaging (MRI) has recently been proposed as a supplementary tool for its diagnosis. A 55-year-old woman presented with back pain and painful proximal weakness of the right leg. Neurologic examination and electrophysiologic studies suggested an upper lumbar plexopathy. MRI disclosed the signal change in lumbar plexus with the atrophy of the innervating muscles. We report a patient with idiopathic lumbar plexopathy confirmed by MRI.