Alzheimer's disease (AD) is the most common neurodegenerative disorder characterized by progressive dementia that ultimately leads to death. Histopathological hallmarks of AD include brain amyloid deposits and neurofibrillary tangles. Major depression is a frequent diagnosis in every gerontopsychiatric clinic that sees patients with both cognitive and affective disorders. Many depressed patients, in fact, are clinically characterized by cognitive impairments. Thus, an assay that excludes - or confirms - probable AD in cognitively impaired patients is desirable. Such assays may use protein markers that are derived from such histopathologically relevant molecules as the amyloid precursor protein (APP) and its derivatives including the amyloid beta-peptides (Abeta). To evaluate the differential diagnostic properties of cerebrospinal fluid (CSF) Abeta and secreted soluble ectodomain (APPs), we quantitated CSF levels of these measures in AD patients and compared them to age-matched control patients with major depression. CSF levels of APPs and Abeta were similar in patients with AD or major depression, and the apolipoprotein E genotype had no influence on CSF levels of Abeta in AD patients. Measurement of Abeta peptide using a novel zinc/copper capture ELISA that detects aggregated Abeta peptides as well demonstrated similar levels in AD and major depression. In AD patients, CSF levels of total Abeta (Abeta1-40 plus Abeta1-42) were inversely correlated with a functional measure of dementia severity (NOSGER), suggesting that CSF levels of Abeta decrease with advancing severity of AD. Thus, CSF levels of Abeta are not useful for the differentiation of AD from major depression. However, CSF levels of Abeta reflect the severity of dementia and may be useful as biological markers of the stage of the disease.