Mitral annular calcium (MAC) may produce mitral stenosis (MS) if its quantity is massive. We define massive MAC as the presence of a huge quantity of calcium underlying the posterior mitral leaflet and extending across all or nearly all of the ventricular aspect of the anterior mitral leaflet. This report was prompted to emphasize the hazards of performing mitral valve replacement in patients with MS secondary to massive MAC. The clinical data and morphology of the operatively excised mitral valves from the 11 patients who had mitral valve replacement for MS secondary to massive MAC are described. Of the 11 patients, 6 died postoperatively, 5 of whom had 4+/4+ MAC. The high mortality in these patients suggests that the decision to perform mitral valve replacement needs to be carefully considered if the quantity of MAC is massive.