Background: Despite latent obstruction(LO, trans-left ventricular outflow tract (LVOT) gradient ���30mmHg with provocation) has been shown to be associated with poorer prognosis in patients with non-obstructive hypertrophic cardiomyopathy (HCM), hemodynamic and geometric determinants of LO has not been fully understood. Method : 35 patients (24 males, 49.4  13.9 years) with non-obstructive HCM (trans-LVOT gradient < 30 mmHg) underwent stepwise supine bicycle exercise echocardiography. Trans-LVOT pressure gradient, mitral geometric parameters, left ventricular ejection fraction (LVEF), and left ventricular end systolic and diastolic dimension (LVESD, LVEDD) were measured at each stage. Result: The highest peak LVOT pressure gradient predominantly occurred within 1 minute after exercise (n=32, 91.3%) rather than during peak exercise (n=3, 8.7%). Significant LO was developed in a total of 9 patients (25%). There was no significant difference in resting maximal septal wall thickness, mitral annular diameter and the leaflets coaptation to septal distance between LO group and non-LO group. LVEF, LVESD and LVEDD did not differ significantly with regard to the patterns of changes between 2 groups. But, patients with LO had longer residual mitral leaflet (defined as residual portions of leaflets after coaptation , 4짹4 vs. 13짹4 mm, p=0.001), higher resting LVOT pressure gradient (7.4짹3.7 vs. 12.9짹5.8 mmHg, p=0.001). Also, substantial decreases in mitral annular diameters from peak exercise to recovery after exercise were observed in LO group, while mitral annular diameters increased after exercise in non-LO group. Conclusion: The highest peak LVOT pressure gradient predominantly occurred within 1 minute after exercise rather than during peak exercise. Decrease of mitral annular diameter, the movement of MV coaptation point toward LV, and longer residual mitral leaflet might be related to LO.