Left ventricular hypertrophy (LVH) is associated with decreased contractile response to inotropic stimulation in animal models, but this has not been documented in humans. To determine whether LVH is associated with decreased myocardial contractile reserve, we measured left ventricular mass, heart rate-corrected velocity of circumferential fiber shortening (Vcfc), end-systolic stress, and LV ejection fraction (LVEF) in patients with LVH and increased end-systolic stress (n = 6) and in patients without LVH (n = 7) who had a normal response to dobutamine stress echocardiography (increased LVEF and no wall motion abnormalities). The afterload-dependent indexes of left ventricular systolic performance were normal at baseline and showed significant increases at peak dobutamine dose (LVH group: Vcfc 0.91 ± 0.11 to 1.76 ± 0.59, p = 0.006; LVEF 49 ± 5 to 65 ± 6, p = 0.001; group without LVH: Vcfc 1.16 ± 0.24 to 1.99 ± 0.36, p = 0.001; LVEF 61 ± 6 to 68 ± 6, p = 0.05). The Vcfc/ end-systolic stress relation, a load-independent index of myocardial contractility, rose in a dosedependent fashion in both groups, but the increment was significantly less for patients with LVH (p <0.02), suggesting a blunted myocardial contractile reserve to inotropic stimulation. The change in heart rate-corrected velocity of circumferential fiber shortening per unit of change in end-systolic stress in each patient at each do-butamine dose showed a linear and inverse relationship. The increment in heart rate-corrected velocity of circumferential fiber shortening for a given reduction in end-systolic stress was larger in patients without LVH than in patients with LVH (p = 0.01). These results suggest that in patients with LVH and increased end-systolic stress, ventricular performance is maintained at the expense of limited myocardial contractile reserve, and that inotropic stimulation unmasks this abnormality, despite a normal response in LVEF and velocity of circumferential fiber shortening. This approach may identify patients with LVH at risk of developing systolic dysfunction and heart failure.