A 36-year-old man developed motor neuron signs consisting of weakness and atrophy of the right upper limb, which progressed to involve the other limbs along with development of upper motor neuron signs including pseudobulbar palsy. He died 8.5 years after onset. Bilateral precentral gyri and putamina were grossly atrophic. In addition to severe degeneration of bilateral pyramidal tracts and marked neuronal cell loss of the precentral gyri and putamina, basophilic inclusions were widely distributed in the motor cortex, putamina, general somatic motor neurons such as the hypoglossal nucleus and spinal anterior horns, and other areas like the red nucleus and inferior olive. The inclusions were clearly shown with Nissl stain to be anilinophilic irregular masses with distinct rims. Ultrastructurally the inclusions appeared to consist of thick filamentous structures of 12-25 nm in diameter studded with electron-dense ribosome-like granules. Thick filamentous profiles were relatively short or occasionally fragmentary, haphazardly mingled with various amounts of granules and other organelles. No prominent accumulation of 10-nm neurofilaments or eosinophilic inclusions like Bunina bodies were found. The inclusions were indistinguishable from those reported in so-called "juvenile" amyotrophic lateral sclerosis.