Flail arm syndrome (FAS), known as one of the atypical amyotrophic lateral sclerosis (ALS) variants, has a similar clinical course and pathologic findings as ALS. Therefore it is difficult to differentiate between ALS and FAS at a glance. There are few reports involving individual analysis of FAS patients to date. The findings of polysomnography (PSG) in patient with FAS are not well known. We report a male FAS patient with review of literatures and several issues related to the diagnostic process.