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Flail arm syndrome (FAS) is a variant of motor neuron disease which is characterized by progressive, predominantly proximal weakness and atrophy of the upper limbs (UL). Because of its heterogeneous presentation and its relatively slow progression, differential diagnosis may be difficult particularly in the early stages of the disease. The aim of this study was to investigate typical clinical features of FAS with special regard to initial symptoms and differences to classical Charcot type amyotrophic lateral sclerosis (ALS). We retrospectively evaluated the clinical features of 42 FAS patients who were seen in the outpatient clinics of 4 German centers between 2000 and 2010 and compared them to 146 sex-matched control patients with classical spinal-onset ALS. FAS patients were younger (54.7 ± 9.3 versus 59.4 ± 12.2 years), male patients were predominantly affected (3.8:1 versus 1.9:1), and FAS patients showed a prolonged survival (53 versus 33 months) compared to classical ALS patients. The share of patients with initial misdiagnoses was 54.8 % and led to ineffective therapy with immunoglobulins in 26 %. Initial symptoms were most frequently present either in distal muscles only or in both proximal and distal muscle groups combined (76 %) and showed an asymmetric distribution pattern in the majority of cases (76 %). Although all patients developed symmetric and predominantly proximal UL weakness and atrophy during the course of their disease, we found that most patients initially showed asymmetric and predominantly distal distribution of symptoms. This may contribute to difficulties in differential diagnosis and to ineffective treatment regimes. [ABSTRACT FROM AUTHOR]