Copper deficiency myelopathy (CDM) is an increasingly recognised mimic of subacute combined degeneration (SCD) of the cord due to cobalamin (vitamin B(12)) deficiency. It has been suggested that copper deficiency induces myelopathy through dysfunction of cytochrome oxidase, which is known to be copper-dependent. However, cytochrome oxidase is not cobalamin-dependent, so this hypothesis fails to explain the phenotypic similarity between CDM and SCD. We propose that the first step in a final common pathway of CDM and SCD is dysfunction of the methylation cycle. This cycle includes both copper and cobalamin-dependent enzymes and catalyses the net transfer of a methyl group from methyltetrahydrofolate to a variety of macromolecules, including myelin proteins. Dysfunction of the cycle might therefore cause failure of myelin maintenance and ultimately myelopathy. One step of the methylation cycle is catalysed by methionine synthase, which is known to be cobalamin-dependent. Nitrous oxide specifically inhibits this enzyme by inactivating methylcobalamin, causing SCD in animals and humans. Both animal and human data suggest that methionine synthase also requires copper, implying that the enzyme may be involved in the pathogenesis of CDM. Another enzyme involved in the methylation cycle, S-adenosylhomocysteine hydrolase, may be regulated by copper. Although this enzyme is not cobalamin-dependent, its potential impairment in copper deficiency may contribute to the overall dysfunction of the methylation cycle. In cases of congenital deficiencies of methylation cycle enzymes, spinal and cerebral demyelination was observed, providing further support for a critical role of the methylation cycle in myelination. Biochemical dysfunction of the methylation cycle has been reported in HIV myelopathy, which has pathological parallels with SCD. This raises the possibility that other demyelinating myelopathies might involve an impairment of the methylation cycle. Our hypothesis could be tested by measuring CSF concentrations of methylation cycle intermediates in cases of CDM, as these reflect spinal cord tissue levels. If it were confirmed, the hypothesis would not only provide a plausible explanation for the phenotypic similarity between CDM and SCD, but might also open up further therapeutic options such as methionine and betaine supplementation.