We investigated the ultrastructure of skein-like inclusions (SI) in 11 patients with motor neuron disease. SI mainly consisted of bundles of filaments. Each filament was approximately 15-25 nm in diameter and had a tubular profile on transverse sections. SI occasionally appeared to contain electron-dense material similar to Bunina bodies (BB), and in clusters of SI, electron-dense material closely resembling BB with vesicles were scattered among the bundles of filaments or were directly attached to the filaments, suggesting a close relationship between SI and BB. We also infer that SI might be the precursor of Lewy body-like hyaline inclusions (LBHI), on the basis of the frequent presence of in-between structures, the coexistence of SI and LBHI, the detection of tubular profiles (component of SI) in LBHI, the far greater frequency with which SI were observed compared with LBHI, and the occasional connection between ubiquitin-positive SI and LBHI observed in preceding immunocytochemical studies.