Ubiquitin (Ub) was determined by a competitive enzyme-linked immunosorbent assay (ELISA) in serum (S) and cerebrospinal fluid (CSF) samples from 29 patients with ‘probable Alzheimer's disease’ (AD), 14 patients with vascular dementia (VAD), and 13 healthy individuals. The mean concentration of Ub in CSF (110 ± 20 ng/mL) was about 20% of that in serum (940 ± 120 ng/mL) in healthy controls. There was no significant correlation between S-Ub and CSF-Ub, or between the CSF/S Ub ratio and the CSF/S albumin ratio. These findings suggest that a major portion of CSF-Ub is intrathecally produced. CSF-Ub was increased while S-Ub was decreased in both AD and VAD patients as compared with controls. As a consequence, the CSF/S Ub ratio showed good discrimination between patients and controls: 22/29 (76%) of the AD patients and 9/14 (64%) of the VAD patients had a CSF/S Ub ratio that was higher than the highest control value. No significant differences in any of the parameters were found between AD and VAD. Ub is involved in an ATP-dependent proteolytic pathway and also acts as a heatshock protein. The increase in CSF-Ub in AD and VAD may therefore be interpreted as a cytoprotective response to abnormal or damaged proteins, and CSF-Ub may have a potential as a non-disease-specific marker for cerebral degeneration.