Two women developed significant neurologic dysfunction, consistent with subacute combined degeneration of the spinal cord, within eight weeks of being anesthetized with nitrous oxide. Investigation revealed that each had preclinical or masked vitamin B12 deficiency. The first patient had two previous ileal resections for Crohn's disease and the other had pernicious anemia. It is postulated that nitrous oxide anesthesia may precipitate neurologic disease in people with unrecognized deficiency of vitamin B12.