Lower motor neurons of the spinal cord of patients with amyotrophic lateral sclerosis (ALS), Werdnig-Hoffmann's disease (WH), X-linked recessive bulbospinal neuronopathy (X-BSNP) and multiple system atrophy (MSA), all of which were known to involve the lower motor neurons, were immunohistochemically examined by using a monoclonal antibody (Ta-51) specific to phosphorylated epitopes of high molecular weight subunits of neurofilaments. The incidence of Ta-51-positive neurons was significantly increased in ALS, WH and MSA, but not in X-BSNP. Ta-51-positive neurons showed a wide variety of morphological appearances, including neurons with normal appearance, central chromatolysis, simple atrophy and neurons containing massive neurofilamentous accumulation. In aged-control cases, similar Ta-51-positive neurons were observed, although to a much lesser extent. In ALS, spheroids and globules, which were strongly positive for Ta-51, were also significantly increased. Ta-51-positive motor neurons, spheroids and globules appeared in proportional to the number of remaining large motor neurons in ALS.