We examined the primary motor area (PMA, Brodmann area 4) from 23 cases of adult-onset sporadic amyotrophic lateral sclerosis (ALS) with immunocytochemistry using anti-glial fibrillary acidic protein antibody. There was astrocytosis in the middle of the pyramidal cell layer in all cases except for one that did not present any upper motor neuron signs clinically. The astrocytosis was characterized by multiple clusters of astrocytes, some of which showed a close association with macrophages. In about a half of the cases, these multiple clusters of astrocytes became confluent and presented as a laminar astrocytosis in the middle of the pyramidal cell layer. Our studies demonstrate a unique pattern of astrocytosis in the PMA in ALS. This pattern of astrocytosis may be useful not only for diagnostic purposes, but also for a better understanding of the pathological process involving the PMA in ALS.