Serum vitamin B12 concentrations were measured in 60 patients undergoing repetitive hemodialysis and in undialyzed patients with chronic renal failure. Dialysis patients had significantly lower serum vitamin B12 levels than the nondialyzed group (312 ± SEM 38 pg/ml versus 793 ± 100), and 19 of 60 dialysis patients had vitamin B12 concentrations less than 200 pg/ml. Folic acid concentration was 5 times greater in dialysis than in nondialysis patients, presumably because the latter received daily supplementation with folic acid. Serum vitamin B12 concentrations fell progressively during the patient's course of dialysis. Neither inadequate dietary intake nor vitamin B12 malabsorption accounted for the differences in the serum vitamin B12 concentrations seen in the two groups. Serum vitamin B12 levels and nerve conduction velocities in 51 dialyzed patients showed a significant correlation. Six dialyzed patients with low serum vitamin B12 levels and slow nerve conduction velocities showed improvement in nerve conduction (+ 14.6 ± 3.3 m/sec) following the parenteral use of pharmacological doses of vitamin B12. The cause of the low serum vitamin B12 concentration is not clear, nevertheless, alterations in serum vitamin B12 seen in some dialysis patients may be a factor in the persistence of abnormal nerve conduction and may be reversed with large doses of parenteral vitamin B12.