Recent studies have revealed extensive axonal damage in patients with progressive multiple sclerosis (MS). Axonal damage can be caused by a plethora of factors including the release of proteolytic enzymes and cytotoxic oxidants by activated immune cells and glia within the lesion. Macrophages and microglia are known to express myeloperoxidase (MPO) and generate reactive oxygen species during myelin phagocytosis in the white matter. In the present study we have measured MPO levels in post-mortem homogenates of demyelinated and non-demyelinated regions of white matter from nine patients with MS and seven controls, and assessed MPO immunoreactivity within MS brain. In homogenates of MS white matter, demyelination was associated with significantly elevated MPO activity when compared to controls. Immunohistochemistry showed MPO to be expressed mainly by macrophages within and adjacent to plaques. Demyelination in MS is associated with increased activity of MPO, suggesting that this production of reactive oxygen species may contribute to axonal injury within plaques.