Phenytoin has been used globally as an effective anticonvulsant. Among its adverse effects, peripheral neuropathy including polyneuropathy has sometimes been reported. We report a case of sensorimotor polyneuropathy associated with high serum level and long-term phenytoin therapy. A 29-year-old male presented with motor weakness in all extremities. He was treated with phenytoin (400 mg/day) for about eight years because of generalized tonic clonic seizure. During nerve conduction assessment, sensorimotor polyneuropathy was discovered.