In this clinical study, the cerebrospinal fluid (CSF) level of a Novemberel form of the β-amyloid peptide (Aβ) extending to position 42 (Aβ42) was determined in patients with Alzheimer's disease (AD) as well as controls. In addition to measurement of CSF Aβ42 levels, total Aβ peptides, microtubule-associated protein τ, and apolipoprotein E (ApoE) genotype were also assessed. It is interesting that CSF Aβ42 levels were found to be significantly lower in AD patients relative to controls, whereas total Aβ levels were not. Aβ42 has recently been shown to preferentially deposit in the brain tissue of patients with AD, suggesting that diminished clearance may account for its reduction in CSF. As previously reported, τ levels were increased in AD patients; however, neither Aβ42 nor τ levels were apparently influenced by the ApoE genotype.