Patients with cobalamin deficiency may experience cognitive impairment or neuropsychiatric symptoms. Although abnormalities of central myelin are the presumed cause of these manifestations, there is a paucity of reports of white matter lesions as shown on neuroimaging studies, and the effects of cobalamin replacement on these lesions are not known. We report a man with subacute cognitive impairment associated with cobalamin deficiency and remarkable confluent white matter abnormalities on MRI, confirmed by biopsy. With cobalamin replacement, both his cognitive deficits and imaging abnormalities partially resolved. This case indicates that leukoencephalopathy, in the absence of anemia or myelopathy, should be added to the spectrum of disorders associated with cobalamin deficiency. Early detection and treatment may be associated with a greater potential for recovery.