An immunohistochemical study was performed to compare glial reactions in recent and old lesions of central pontine myelinolysis (CPM). Regions of demyelination and destruction of oligodendrocytes, showed reduced immunoreactivity of myelin basic protein (MBP), myelin-associated glycoprotein (MAG), transferrin, and carbonic anhydrase C (CA C). In addition, labeling of glial fibrillary acidic protein (GFAP) and S-100 protein revealed distinct dystrophic alterations of the astroglia. Remarkably, immunolabeling of GFAP was drastically reduced in astrocytic cytoplasm within freshly demyelinated lesions. Immunostaining of vimentin revealed a differential intracytoplasmic decoration of hypertrophic and dystrophic astrocytes in recent and old CPM lesions. Immunolabeling of desmin failed to stain glial cells. Monoclonal antibodies against HNK-1 exhibited greatly increased immunoreactivity both of persisting oligodendrocytes and of reactive fibrillary astrocytes in old CPM foci. In freshly demyelinated lesions, enhanced immunoreactivity of the X-hapten (3-fucosyl-N-acetyllactosamine) was prominent in astroglia and oligodendrocytes. Simultaneously, reactive astrocytes revealed intracytoplasmic labeling of laminin. Quantitation of GFAP+ astroglia in fresh CPM and control cases revealed an increase in the number of astrocytes within the demyelinated foci and in the surrounding non-demyelinated pontine tissue of CPM cases. The occurrence of astroglial alterations in the demyelinated foci of CPM could be interpreted as "astroglial dystrophy" which may represent a pathogenic factor in CPM. Furthermore, it is possible that changes of the glial microenvironment may influence the astroglia to revert transiently back to an immature phenotype as indicated by the enhanced expression of the X-hapten and HNK-1, and the de novo synthesis of vimentin and laminin.