In heart failure, diastolic dysfunction is responsible for about 50% of the cases, with higher prevalence in women and elderly persons and contributing similarly to mortality as systolic dysfunction. Whereas the cardiac systolic diagnostics in ventricular assist device patients from pump parameters have been investigated by several groups, the diastolic behavior has been barely discussed. This study focuses on the determination of ventricular relaxation during early diastole in rotary blood pump (RBP) recipients. In conventional cardiology, relaxation is usually evaluated by the minimum rate and the time constant of left ventricular pressure decrease, dP/dtmin and τP. Two new analogous indices derived from the pump flow waveform were investigated in this study: the minimum rate and the time constant of pump flow decrease, dQ/dtmin and τQ. The correspondence between the indices was investigated in a numerical simulation of the assisted circulation for different ventricular relaxation states (τP ranging from 24 to 68 ms) and two RBP models characterized by linear and nonlinear pressure-flow characteristics. dQ/dtmin and τQ always correlated with the dP/dtmin and τP, respectively (r > 0.97). These relationships were influenced by the nonlinear pump characteristics during partial support and by the pump speed during full support. To minimize these influences, simulation results suggest the evaluation of dQ/dtmin and τQ at a pump speed that corresponds to the borderline between partial and full support. In conclusion, at least in simulation, relaxation can be derived from pump data. This noninvasively accessible information could contribute to a continuous estimation of the remaining cardiac function and its eventual recovery.