We have recently reported that, in addition to the widespread occurrence of ubiquitinated neuronal intranuclear inclusions (NIIs), the restricted occurrence of ubiquitinated intracytoplasmic filamentous inclusions in the neurons of the cerebellar dentate nucleus (CDN) is a characteristic feature of dentatorubral-pallidoluysian atrophy (DRPLA). Interestingly, these neuronal intracytoplasmic filamentous inclusions (NIFIs) were morphologically indistinguishable from the skein-like inclusions (SLIs) described previously in the spinal anterior horn cells in amyotrophic lateral sclerosis (ALS). In the present study, we examined immunohistochemically the CDN in ten patients with clinicopathologically and genetically confirmed DRPLA and the spinal anterior horns in five patients with sporadic ALS, using a monoclonal antibody (1C2) directed against long polyglutamine stretches. In all of the patients with DRPLA, both the NIFIs and the NIIs were visualized clearly with 1C2. Conversely, in the patients with ALS all structures, including the SLIs, were completely negative. These findings indicate that in DRPLA, the NIFIs in the CDN are an alteration that is directly related to the causative gene abnormality (an expanded CAG repeat encoding polyglutamine) and that, from the molecular point of view, they are distinct from the SLIs in ALS.