Chronic ischemic mitral regurgitation (CIMR) is poorly understood and repair operations are often unsatisfactory. This study elucidates the mechanism of CIMR in an ovine model. Sonomicrometry array localization measured the three-dimensional geometry of the mitral annulus and subvalvular apparatus in five sheep before and 8 weeks after a posterior infarction of the left ventricle that produced progressive severe CIMR. End systolic annular area increased from 647 ± 44 mm 2 to 1,094 ± 173 mm 2 ( p = 0.01). Annular dilatation occurred equally along the anterior (47.0 ± 5.6 mm to 60.2 ± 4.9 mm, p = 0.001) and posterior (53.8 ± 3.1 mm to 68.5 ± 8.4 mm, p = 0.005) portions of the annulus. The tip of the anterior papillary muscle moved away from both the anterior and posterior commissures by 5.2 ± 3.2 mm ( p = 0.021) and 7.3 ± 2.2 mm ( p = 0.002), respectively. The distance from the tip of the posterior papillary muscle to the anterior commissure increased by 11.0 ± 5.7 mm ( p = 0.032) while the distance from the tip of the posterior papillary muscle to the posterior commissure remained constant. Progressive dilatation of both the anterior and posterior mitral annuli, increased annular area, and asymmetric ventricular dilatation combine to cause CIMR by distortion of mitral valve geometry and tethering of leaflet coaptation. Therefore complete ring annuloplasty may be superior to partial annuloplasty in the treatment of CIMR.