Long-term deficiency of cobalamin or folate causes a demyelinating disease of the brain and spinal cord. A reduced supply of methyl groups has been implicated as its cause. To examine the mechanisms of demyelination in human beings, we have studied three children with sequential inborn errors of the methyl-transfer pathway. One child had abnormal methylfolate metabolism, one abnormal methylcobalamin metabolism, and one hypermethioninaemia probably caused by methionine adenosyltransferase deficiency. Magnetic resonance imaging of the brain and measurement of cerebrospinal-fluid concentrations of 5-methyltetrahydrofolate, methionine, and S-adenosylmethionine were carried out before and after 6-12 months of appropriate treatment. Each patient had abnormal myelination before treatment; the scans suggested demyelination. The only consistent biochemical abnormality in the cerebrospinal fluid was a low concentration of S-adenosylmethionine. Treatment led to substantial clinical improvement, apparent remyelination, and increases in cerebrospinal-fluid S-adenosylmethionine concentration into the normal range. Cerebrospinal-fluid concentrations of S-adenosylmethionine and methionine were significantly lower in eight other children with errors of the methyl-transfer pathway than in an age-matched reference population (mean [95% confidence interval] standard deviation score -1.81 [0.57], p less than 0.001 for S-adenosyl methionine and -1.82 [0.19], p less than 0.001 for methionine). The concentrations of these metabolites increased to within the reference range on treatment. We have shown that demyelination is associated with cerebrospinal-fluid S-adenosylmethionine deficiency and that restoration of S-adenosylmethionine is associated with remyelination.