Central pontine myelinolysis (CPM) is a demyelinative disorder of unknown origin. Recent clinical and experimental studies have indicated an association of CPM with a rise in the serum sodium level. I propose that the rapid rise in the serum sodium level causes an osmotic injury to the endothelium resulting in the release of myelinotoxic factors and/or the production of vasogenic edema. The latter factors may lead to demyelination. The patient at risk, viz, a chronically ill, alcoholic, cirrhotic person, may be the one least able to generate protective cerebral mechanisms against the osmotic stress. The location of lesions may be explained by a suitable anatomic arrangement consisting of an extensive admixture of gray and white matter; thus, myelinotoxic factors derived from the richly vascular gray are able to interact with adjacent bundles of myelin-containing white matter.