We report a case of sporadic amyotrophic lateral sclerosis (ALS) characterized by a marked accumulation of neurofilaments in the cytoplasm of neurons. The neurofilament was identified by immunohistochemical and electron microscopic studies. The distribution of the accumulation in this case was unique, not only in the motoneurons of the anterior horn but also in the neurons of the other areas of the spinal gray matter, some nuclei in the brain stem, pontine reticular formation, substantia nigra, dentate nucleus in the cerebellum and pyramidal cells in the motor cortex. These observations shed light on the pathogenesis of ALS.