Dermatomyositis (DM) is an idiopathic inflammatory myo-pathy, characterized by inflammation of the proximal skel-etal muscles and typical skin manifestations, which results in symmetric muscle weakness. A 43- year-old man was presented with skin rash and left leg weakness, and he had a history of poliomyelitis. Initially, he was diagnosed as having post-polio syndrome (PPS) due to unilateral muscle weakness and a result of an the electromyography (EMG), which had shown patterns of PPS. After 4 months with conservative therapy for PPS, weakness of bilateral upper arms had developed and skin rashes on his entire body had aggravated and progressed. He was diagnosed as hav-ing dermatomyositis, based on elevated muscle enzyme lev-els, typical skin rashes, and typical EMG findings, which indicated muscle disease. When a patient with previous po-liomyelitis has a newly developed muscle weakness or pain, we should consider various possible causes other than PPS.

Keywords: Dermatomyositis, Post-polio syndrome, Poliomyelitis, Skin rash, Muscle weakness