In 2 of 16 cases with sporadic amyotrophic lateral sclerosis (ALS) large numbers of axonal swellings were observed in the corticospinal tracts over a region extending from the posterior limbs of internal capsules to the bulbar pyramids. On electron microscopy, these axonal swellings were seen to consist of accumulations of neurofilaments and altered neuronal organelles (mitochondria and secondary lysosomes). Their morphology differed from the spheroids seen in the anterior horn in ALS.