Various C-terminally truncated amyloid β peptides (Aβ) are linked to Alzheimer's disease (AD) pathogenesis. Cerebrospinal fluid (CSF) concentrations of Aβ38, Aβ40, and Aβ42 were measured by enzyme-linked immunosorbent assay in 30 patients with AD and 26 control subjects. CSF Aβ42 levels was decreased in patients with AD, whereas CSF Aβ38 and Aβ40 levels were similar in patients with AD and control subjects. All three Aβ peptides were interrelated, particularly CSF Aβ38 and Aβ40. Diagnostic accuracy of CSF Aβ42 concentrations was not improved by applying the ratios of CSF Aβ42 to Aβ38 or Aβ40.