Making a diagnosis of typical amyotrophic lateral sclerosis (ALS) is not a tough job, but when it comes to atypical forms of motor neuron disease (MND) which are not uncommon in clinical setting, we may have some difficulty to diagnose ALS/MND. There is striking phenotypic variation in sporadic ALS/MND, such as frail arm syndrome (brachial amyotrophic diplegia), pseudopolyneuritic form, hemiplegic type, ALS/MND with markedly extended involvement beyond the motor system, and MND with basophilic inclusion bodies. These variations must be recognized if physicians are to tailor advice on disease progression, prognosis, drug therapy, and care to the needs of the individual. Clinical trials of new therapeutic agents have been performed, on the assumption that patients with ALS/MND have the same underlying etiology, addressing the heterogeneous population of the patients under a single diagnostic category. This can be detrimental to the well-being of the individual, because clinical heterogeneity may mask drug effects in clinical trials. The attempt to categorize subgroups based on the clinical and pathological backgrounds within the spectrum of ALS/MND may be a critical step in facilitating clinical research in ALS/MND. Definition of clinicopathologic syndromes in patients with ALS/MND is an important challenging task that cannot be ignored.