A 7-year-old child has been studied, who had an error of vitamin B 12 metabolism leading to intracellular deficiency of cobalamins, including the two coenzyme forms, methyl- and adenosyl-cobalamin. Her brain showed multifocal areas of demyelination and vascular damage in the white matter of the centrum ovale. Chemical analysis showed loss of ethanolamine phospholipids and sphingomyelin from the white matter, and a marked fall in the content of unsaturated fatty acids in all phospholipids in grey matter. There were also abnormal amounts of odd chain fatty acids (C 15 and C 17) in the phosphatidyl choline fraction of white matter and, uniquely for grey matter, also in the globus pallidus. The neuropathological findings are similar to those of classical vitamin B 12-deficiency, including the vascular lesions. The genesis of the abnormalities of lipid composition is uncertain.