One of the pathological features in Alzheimer's disease (AD) is neurofibrillary tangles in the brain. The main constituent of tangles is the microtubuli-associated protein tau in a hyperphosphorylated state. Tau is also released into cerebrospinal fluid (CSF), and in this study we have used an enzyme linked immunosorbent assay to measure tau in CSF from AD and control cases. Our findings show that tau levels in AD cases are significantly elevated compared to healthy control individuals. We suggest that tau may serve as a biochemical marker of Alzheimer's disease.