Two patients with asymptomatic vitamin B(12) deficiency developed a peripheral neuropathy 6-7 weeks after exposure to nitrous oxide during general anaesthesia, and 3 and 11 days after receiving cyclophosphamide. The serum B(12) concentration was less than 20 pmol/l and the symptoms resolved after B(12) replacement. We believe that the neurological symptoms were triggered by exposure to nitrous oxide. A possible novel feature in these patients was the close temporal relationship between the onset of the symptoms and cyclophosphamide treatment. It is possible that cyclophosphamide reduced the availability of remaining B(12) stores.