A 26-year-old male patient gradually developed muscular atrophy of the right lower leg over a two-year period. Neurological examination revealed absent Achilles tendon reflex and muscular atrophy of the right lower leg and right hamstring muscles. Conduction velocity of the F waves was delayed in the right posterior tibial nerve. A computerized tomography scan and magnetic resonance imaging revealed a mass lesion along the proximal segment of the right sciatic nerve. Exploration revealed a fusiformly swollen sciatic nerve. Histological examination showed that a swollen segment of the sciatic nerve was filled with onion-bulb formations of perineurial cells, consistent with the diagnosis of localized hypertrophic neuropathy. This condition should be added to several etiologies of monomelic amyotrophy. Electrophysiological studies and neuroimaging techniques were useful in obtaining differential diagnosis.