The 13 cases of methylcobalamin (MeCbl) deficiency presenting in early infancy have all been developmentally delayed, and the majority have had seizures, hypotonia, lethargy, and microcephaly. The CNS injury appears to occur during the first 6 months of postnatal life. The same symptoms are seen in acquired cobalamin (Cbl) deficiency in the same age group. MRI performed at age 18-19 months and after 13-14 months of large amounts of Cbl, in two cases showed delayed myelination, most pronounced in the cerebrum. Isolated MeCbl deficiency is the consequence of cblE and G mutations where the lesion is of a single Cbl-dependent enzyme, the methyltransferase. One effect of a deficiency of MeCbl, and of the associated failure of the methionine synthase reaction, is, therefore, an impairment of myelination of the brain of the newborn. The slow, but usually incomplete, improvement in psychomotor status after years of treatment with Cbl may be related to the eventual myelination. However, the hypotonia, lethargy, and impaired responsiveness react to treatment with Cbl within 24-48 hours, which suggests an expression of MeCbl deficiency on the CNS distinct from the delayed myelination. Although there is much to be learned, it is now clear that a normally functioning Cbl-dependent methyl transferase is required for development and function of the human brain.