Neuronal thread protein is a recently characterized, approximately 20-kd protein that accumulates in brains with Alzheimer's disease (AD) lesions. This study examined whether concentrations of neuronal thread protein (NTP) were also increased in the cerebrospinal fluid (CSF) of individuals with probable (clinically diagnosed) and definite (histopathologically proved) AD. Using a highly sensitive three-site monoclonal antibody-based immunoradiometric assay, we measured NTP concentrations in CSF from 84 patients with probable AD and mild dementia (duration, 4.05 +/- 0.36 years), 45 with Parkinson's disease and minimal or no dementia (duration, 4.73 +/- 0.78 years), 73 with multiple sclerosis, and 73 nondemented control subjects. NTP concentrations were also measured in postmortem ventricular fluid and temporal lobe neocortex extracts from 31 subjects with histopathologically proved AD and 14 age-matched control subjects. The mean concentration of NTP in the CSF was higher in AD (4.15 +/- 0.25 ng/ml; 95% confidence interval [CI] limits, 3.65-4.65) than in Parkinson's disease (1.96 +/- 0.16 ng/ml; 95% CI, 1.65-2.27), multiple sclerosis (1.6 +/- 0.14 ng/ml; 95% CI limits, 1.33-1.88), or control subjects (1.27 +/- 0.06 ng/ml; 95% CI limits, 1.15-1.40) (p < 0.001). In addition, 70% of the patients with probable AD had concentrations of NTP in CSF that were higher than 2.5 ng/ml (> upper 99% CI limit in the control group), compared with 23% of Parkinson's disease patients, 11% of multiple sclerosis patients, and 4% of control subjects. The mean concentrations of NTP in the ventricular fluid and brain tissue from individuals with documented AD and end-stage dementia were threefold higher than the levels detected in the CSF from the remaining patients with probable AD and mild dementia. Moreover, of 9 patients with AD, postmortem brain and CSF manifested 5- to 50-fold higher levels of NTP compared with the CSF samples obtained an average of 6 years earlier. These findings demonstrate that NTP levels are elevated in the CSF of individuals with AD and that NTP levels in the CSF increase strikingly with progression of dementia and neuronal degeneration.