As clinical diagnosis of Alzheimer's disease is only 80%-90% accurate, there is a need to identify biochemical markers of Alzheimer's disease. Previous studies have shown an abnormality in the glycosylation of acetylcholinesterase (AChE) in the CSF collected postmortem from patients with Alzheimer's disease. This abnormality was very specific for Alzheimer's disease, as it was not detected in other illnesses causing dementia. We report here that the glycosylation of AChE is also altered in lumbar CSF collected antemortem. The altered glycosylation was due to increased concentrations of a minor AChE isoform that does not bind to concanavalin A (Con A). Glycosylation of AChE may eventually be of diagnostic value, especially when used in combination with other CSF markers.