Background: Cardiac contractility modulation (CCM) by non-excitatory electrical current is an effective therapy in chronic heart failure. Transsarcolemal calcium transients contribute largely to the observed contractile benefit. As a rise in diastolic calcium concentrations could influence myocardial diastolic function, we examined active isovolometric relaxation during CCM stimulation. Methods: In total 26 patients were implanted with a CCM-generator. Maximum rate of active isovolumetric relaxation during ventricular diastole (-dP/dt) was calculated in 5 selected patients with significant gain of systolic capacity (Mean +dP/dt: +12.1%). Results: A significant increase of active relaxation during CCM-stimulation was found in 3 of 5 patients (-dP/dt: I. +6.1%; II. +7.1%; III. +12.7%). Two patients showed no significant effect (-dP/dt: IV. +0.7%; V. +1.0%). Thus CCM-stimulation could lead to an improved overall diastolic myocardial performance, though the hemodynamic impact of CCM-stimulation during diastole needs to be further defined. Conclusion: Though beneficial systolic CCM-effects are thought to be mediated by enhanced systolic calcium transients, a deterioration of active diastolic relaxation was not detected in any patient. Moreover the observed increase of active relaxation could improve overall diastolic function in patients with documented systolic response of CCM-stimulation.