Patients with subclinical cobalamin deficiency may develop a subacute myeloneuropathy after nitrous oxide anesthesia. Frequently they have hematologic manifestations, however there is only a few reports of spinal lesions shown on MR and particularly of the effects of cobalamin replacement therapy on these lesions. We describe a patient with unsuspected cobalamin deficiency who developed subacute combined degeneration of the spinal cord after nitrous oxide anesthesia. MR showed high intensity in dorsal columns on T2-weighted image, and posterior resolution of abnormalities after therapy. We discuss the importance of preoperative hematologic manifestations for select high risk patients and prevent this complication. On the other hand, we consider MR as a useful tool to improve our knowledge in the pathophysiology of the neurologic manifestations of the process.