Postoperative cardiac function was evaluated by dynamic exercise echocardiography in 36 cases with mitral stenosis, and electron microscopic examination of the LV myocardial cells was also carried out. Ultrastructual changes of the LV myocardial cells were graded into scores (EM-score). EM-scores correlated with % FS (r=-0.54), mVcf (r=-0.60) at rest and % FS (r=-0.54), mVcf (r=-0.50) after exercise. In 36 cases, 25 cases underwent conventional MVR (group Ⅰ) and 11 underwent MVR with preservation of the posterior mitral apparatus (group Ⅱ). When postoperative cardiac function was compared in 13 cases less than 4 of EM-score in group Ⅰ because EM-scores were less than 4 in all cases in group Ⅱ, no difference was observed in two groups except for mVcf at rest. The vaulues of % △FS and % △mVcf in group Ⅰ were better than those in group Ⅱ. This suggests that preservation of the hypertrophic and sclerotic subvalvular tissue in mitral stenosis disturebs the LV diastolic wall motion.