The recent development of acetylcholinesterase inhibitors to treat patients with Alzheimer's disease has increased interest in the use of biochemical markers for the early detection and diagnosis of dementia, but only the measurement of the protein 14-3-3 in cerebrospinal fluid (CSF) to help diagnose sporadic Creutzfeldt-Jakob disease has become accepted clinical practice. CSF concentrations of tau protein and beta-amyloid peptide 42 have been widely investigated as potential diagnostic tests for Alzheimer's disease, but neither has shown sufficient sensitivity and specificity for clinical use. Preliminary investigations suggest that beta-amyloid peptide 42 may be useful in monitoring disease progression, but this needs to be verified. In addition, biochemical investigations may help to identify the small number of patients with treatable causes of dementia such as hypothyroidism and vitamin B12 deficiency, as well as any other compounding condition such as anaemia or diabetes mellitus that increase morbidity.