Abstract— Alzheimer's disease is a progressive degenerative dementia characterized by the abundant presence of neurofibrillary tangles in neurons. This study was designed to test whether the microtubule-associated protein, a major component of neurofibrillary tangles, could be detected in CSF. Additionally, we investigated whether CSF levels were abnormal in Alzheimer's disease as compared with a large group of control patients. We developed a sensitive sandwich enzyme-linked immunosorbent assay using AT120, a monoclonal antibody directed to human, as a capturing antibody. With this technique, the detection limit for was less than 5 pg/ml of CSF. Using ATS, which recognizes abnormally phosphorylated ser-ines 199–202 in, the detection limit was below 20 pg/ml of CSF. However, with AT8, we found no immunoreactiv-ity in CSF, suggesting that only a small fraction of CSF contains the abnormally phosphorylated AT8 epitope. Our results indicate that CSF levels are significantly increased in Alzheimer's disease. Also, CSF levels in a large group of patients with a diversity of neurological diseases showed overlap with CSF levels in Alzheimer's disease.