Objective To investigate the clinical and pathological features of flail arm syndrome (FAS).Methods The clinical data of 90 patients with amyotrophic lateral sclerosis (ALS) were collected, and cases of FAS were screened out. The clinical manifestations, laboratory examination results, electrophysiological results, and pathological characteristics (based on biopsy) of skeletal muscles were summarized and analyzed.Results Eight of ALS patients were diagnosed with FAS. The main symptoms of FAS were symmetric weakness and atrophy of bilateral upper extremities. Serum level of creatine kinase was normal or increased mildly or moderately. The electromyography showed that 3 or more innervated areas of the four segments of spinal cord had fibrillation and positive wave, with broadened and heightened motor unit action potential. The main pathological characteristics included small angular fibers, disorder of myofibrillar network, and target fibers.Conclusions FAS is a clinical variant of ALS. Electrophysiological examination can detect subclinical spinal cord involvement and helps the diagnosis and differential diagnosis of FAS.