Typical amyotrophlc lateral sclerosis (ALS)/motor neuron disease (MND) is not hard to diagnose, but we may face some difficulties in differentiating between atypical forms of ALS/MND which account for about 20% and other non-ALS diseases. There is striking phenotypic variation in sporadic ALS/MND, i.e. flail arm syndrome (brachial amyotrophic diplegia), pseudopolyneuritic form, hemiplegic type, ALS/MND with markedly extended involvement beyond the motor system, spinal progressive muscular atrophy, primary lateral sclerosis, progressive bulbar palsy, and MND with dementia. These variations must be recognized when physicians are to tailor advice on disease progression, prognosis, 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 background may be a critical step in facilitating clinical research in ALS/MND. It will be necessary to elucidate whether various kinds of subgroups of motor neuron diseases lie on the same spectrum of ALS.