The abnormal polymerization of the tau molecule into insoluble filaments is a seminal event in the neurodegenerative process underlying Alzheimer's disease. Previous experimentation has shown that the microtubule-binding repeat region of the molecule is vital for its ability to polymerize in vitro into filaments similar to those found in Alzheimer's disease. However, it is becoming clear that regions outside the microtubule-binding repeat, such as exons 2 and 3 and the carboxy-terminal tail, can greatly influence its polymerization. Since it has been previously postulated that the amino terminus of tau could be involved in generating pathological conformations in the disease state, its role in the polymerization process was investigated. This report demonstrates that the removal of the amino terminus greatly inhibits the polymerization of the tau molecule, reducing both the rate and extent of polymerization. These results support the hypothesis that the ability of tau to form specific conformations involving the amino terminus is an early event in the formation of tau polymers in the disease state. Furthermore, the mutation of arginine 5 to leucine (R5L), mimicking an amino-terminal tau mutation found in a single case of FTDP-17, enhances the polymerization of the tau molecule. Therefore, the amino terminus of the tau molecule, while largely overlooked in studies of its polymerization, is a significant contributor to the polymerization process.