Nerve conduction studies and sural nerve biopsy were performed on three patients with vitamin B12 deficiency and symptoms of peripheral neuropathy. The pathological findings were those of axonal degeneration; there was no evidence of demyelination. The patients were reviewed at intervals of 5-15 years commencement of treatment; progression of the neuropathy had been arrested by treatment, but in all cases residual neurological abnormalities persisted. In one patient with autonomic neuropathy, the postural hypotension resolved rapidly and fully with treatment.