Myeloperoxidase (MPO) is an enzyme stored in azurophilic granules of polymorphonuclear neutrophils and macrophages and released into extracellular fluid during inflammatory processes. Several studies have shown its involvement into oxidative stress and inflammation. Recently, MPO has been considered its role as a possible marker of plaque instability and a useful tool for the prognostic evaluation of patients with coronary artery disease. Aim of this review is to provide an overview of patophysiological, analytical and clinical characteristics of MPO and to summarize the evidence about its usefulness as diagnostic and prognostic marker in the setting of acute coronary syndrome.