CSF levels of proteins related to the lesions of Alzheimer's Disease (AD) may be informative. These include the microtubule-associated protein tau, an integral component of neurofibrillary tangles, and A beta, a 4kDa protein that accumulates in senile plaque amyloid. Many studies have found that CSF tau is increased in AD compared to normal controls (NC). CSF tau may be increased in a minority of patients with destructive neurological disorders or several neurodegenerative conditions, making its use in differential diagnosis less clear. CSF tau consists of fragments that lack extensive phosphorylation. CSF levels of A beta species ending at residue 40 are unchanged in AD. However species ending at residue 42 (A beta 42) are significantly decreased in AD compared to NC. Decreased A beta 42 may be found in patients with other dementias, some of whom may harbor AD pathology. Simultaneous measurement of CSF A beta 42 and tau may improve discrimination between AD and NC, and may facilitate the diagnosis of early stage AD.