Myeloperoxidase (MPO) is a glycosylated and heme-containing enzyme of approximately 80 kDa. The enzyme is stored in azurophilic granules of polymorphonuclear neutrophils and macrophages, and is released into extracellular fluid in the setting of inflammatory processes. MPO participates in the innate immune defense mechanism through formation of microbicidal reactive oxidants and diffusible radical species. Its involvement in oxidative stress and inflammation makes it a possible marker for atherosclerotic plaque instability and a clinical tool for the evaluation of coronary heart disease.