Iron as well as aluminum is reported to accumulate in neurons with neurofibrillary tangles (NFTs) of Alzheimer's disease (AD) brain. Previously we demonstrated that aluminum (III) shows phosphate-dependent binding with hyperphosphorylated τ (PHFτ), the major constituent of NFTs, thereby inducing aggregation of PHFτ. Herein we report that iron (III) can also induce aggregation of soluble PHFτ. Importantly, for the aggregation of PHFτ to occur, iron in the oxidized state (III) is essential since iron in the reduced state (II) lacks such ability. Furthermore, iron (III)-induced aggregation is reversed by reducing iron (III) to iron (II). Thus the iron-participating aggregation is mediated not only by τ phosphorylation but also by the transition of iron between reduced (II) and oxidized (III) states. Further incubation of insoluble PHFτ aggregates isolated from AD brain with reducing agents produced liberation of solubilized PHFτ and iron (II), indicating that PHFτ in association with iron (III) constitutes the insoluble pool of PHFτ. These results indicate that iron might play a role in the aggregation of PHFτ leading to the formation of NFTs in AD brain.