Widespread astrogliosis exists in the subcortical white matter in amyotrophic lateral sclerosis (ALS). As revealed by glial fibrillary acidic protein (GFAP) immunostaining, the gliosis has the morphological properties of an active process. It is present in the midfrontal, inferior parietal, temporal, cingulate, and occipital cortices, as well as in the motor cortex. Compared to matched regions from other neurological diseases, the gliosis in ALS does not appear to be the nonspecific result of a progressive, degenerative disease. In cell number and apparent cell size, the gliosis is comparable to that present in neurological diseases known to have white matter gliosis. Cytologically, the gliosis most closely resembles that present in cases of cerebral infarction. The basis for this similarity is unknown.