This chapter discusses the neurological features of 'classical' and 'sub-clinical' B12 deficiency and explores possible reasons why they might differ. It also discusses high prevalence of subclinical deficiency in older patients, along with its association with neurodegenerative diseases. The chapter presents the neurological features of clinical and subclinical deficiency, and discusses how these two apparently disparate conditions might be reconciled. The presence of neurological features in B12-deficient patients was not related to the overall severity of deficiency of the vitamin, leading to the conclusion that, in most patients, either neurologic or haematologic dysfunction predominates. The link between metabolic B12 deficiency and dementia prompted studies of its possible association with other chronic neurological diseases. C. J. M. Van Tiggelen demonstrated that in some patients with senile dementia of Alzheimer-type there was a high incidence of pathologically low levels of B12 in cerebrospinal fluid (CSF) despite normal serum levels.