A 64-year-old male with diabetes mellitus had a history of abdominal surgeries due to gallstones and appendicitis. There was no identifiable cause of his PN palsy such as orthopedic procedures or leg trauma. Seven years earlier he suffered left leg-and low back pain for which he underwent two lumbar posterior decompression procedures at a local hospital. His lower limb pain decreased, however, numbness in the left L5 area persisted. Two years before admission to our hospital his left leg pain gradually reappeared. He was conservatively but unsuccessfully treated elsewhere with medications including nonsteroidal anti-inflammatory drugs, and later he came to our hospital.On admission, he reported numbness from the left lower thigh to the dorsum of the foot at rest. Walking exacerbated the symptoms and elicited pain in that area. He was unable to walk more than 100 m and experienced intermittent claudication. There was slight motor weakness of the ankle dorsiflexor at rest; it was gradually increased by walking and he stumbled easily. The Tinel sign test over the PN was not informative and lumbar magnetic resonance imaging (MRI) study showed adequate decompression of the left L5 root (Fig. 1). The clinical symptoms led us to suspect PNEN despite the absence of an identifiable cause at onset or of an obvious inciting event.We performed electrophysiological testing to measure the motor nerve conduction velocity (MNCV) of the PN. Stimulation applied proximal (head of the fibula) and distal (ankle) to the entrapment point elicited responses at the extensor digitorum brevis muscle. The MNCV proximal and distal to the entrapment point was 51.2 m/s, indicating that there was no conduction block on the PN in the at-rest position. He did not undergo electromyogram (EMG) or sensory nerve conduction (SNC) studies.His symptoms failed to improve under observation therapy and their severity affected his activities of daily living. To identify the origin of the intermittent symptoms we observed his ankle plantar flexion during walking. As a PNEN provocation test we also performed loading of the repetitive ankle plantar flexion in the at-rest position to avoid the lumbar factor (Fig. 2). His symptoms appeared reproducibly within 10 s of loading.Using a microscope and no proximal tourniquet, we performed left PN neurolysis under local anesthesia.5) We made a 3-cm oblique skin incision behind the fibular head and proceeded antero-inferiorly along the PN. This revealed the The diagnosis of peroneal nerve (PN) entrapment neuropathy (PNEN) is based on clinical symptoms and nerve conduction studies. However, these studies do not always detect PNEN. Our 64-year-old patient suffered persistent left L5 numbness after two lumbar surgeries. Two years before admission to our institute his left leg pain gradually reappeared. When walking, his numbness in the left lower thigh to the dorsum of the foot increased. Electrophysiological testing revealed no conduction block on the PN. To identify the origin of his intermittent symptoms we performed ...