Background Diastolic dysfunction in long-term heart failure is accompanied by abnormal neurohormonal control and ventricular stiffness. The diastolic phase is determined by a balance between pressure gradients and intrinsic ventricular wall properties: according to a mathematical model, the latter (ie, left ventricular [LV] elastance, K LV) may be calculated by the formula: K LV = (70/[DT-20]) 2 mm Hg/mL, where DT is the transmitral Doppler deceleration time. Methods and Results In 54 patients with chronic systolic heart failure (39 men, 15 women; age 65 ± 10 years; New York Heart Association [NYHA], 2.3 ± 0.9; ejection fraction [EF], 32% ± 5%), we analyzed the relationship between K LV and an index of neurohormonal derangement (levels of brain natriuretic peptide [BNP]), and investigated whether 3 months of physical training could modulate diastolic operating stiffness. Patients were randomized to physical training (n = 27) or to a control group (n = 27). Before and after training, patients underwent Doppler echocardiogram and cardiopulmonary stress test. At baseline, ventricular stiffness was related to BNP levels ( P < .01). Training improved NYHA class, exercise performance, and estimated pulmonary pressure. BNP was reduced. Ventricular volumes, mean blood pressure, and EF remained unchanged. A 27% reduction of elastance was observed (K LV, 0.111 ± 0.044 from 0.195 ± 0.089 mm Hg/mL; P < .01), whose magnitude was related to changes in BNP ( P < .05) and to K LV at baseline ( P < .01). No changes in K LV were observed in controls after 3 months (0.192 ± 0.115 from 0.195 ± 0.121 mm Hg/mL). Conclusions In heart failure, left ventricular diastolic stiffness is related to neurohormonal derangement and is modified by physical training. This improvement in LV compliance could result from a combination of hemodynamic improvement and regression of the fibrotic process.