Tachycardia is the most conspicuous finding associated with exertion. The mechanism of exertional dyspnea in patients(Pts) with relaxation abnormality(RA) has not been clearly elucidated. In this study, we evaluated the hemodynamic effects of tachycardia in Pts with RA. Eleven Pts with stable RA, in whom mitral inflow patterns did not change during the leg raising maneuver(Gr1), and 8 subjects with normal mitral inflow pattern (Gr2) were enrolled. Left ventricular mean diastolic pressure(LVmDP), LV dimensions, stroke volume(SV), and LV ejection(LVET), filling(LVFT) and isovolumic times between right atrial pacing rates of 80/min and 120/min were compared. Both Gr1 and 2 showed significant decreases in LVmDP when HR increased(Figure1). At a HR of 80/min, no significant difference was noted between Gr1 and 2 with respect to SV(51.4짹13ml vs.45.2짹9ml,p=0.35).However, a decrement in SV between a HR of 80/min and 120/min was greater in Gr1(21.2짹9ml vs. 5.1짹7.2ml,p=0.001). Therefore, Gr1 showed a significantly lower SV(30.2짹7.1ml vs. 40.1짹6.9ml,p<0.05) at a HR of 120/min(Figure2). In terms of time intervals between HRs of 80/min and 120/min, Gr1 showed a significantly greater reduction in LVET(84.5짹20.1ms vs. 30.0짹34.6ms,p<0.005) and a smaller reduction in LVFT (106.4짹38.5ms vs. 166.3짹30.7ms,p<0.005) than Gr2. In conclusion, results of our study suggest the role of an inadequate SV response to tachycardia, probably due to a greater reduction in LVET, as an important mechanism of exertional dyspnea in patients with RA.