Aims To assess the impact of left ventricular (LV) volume/mass ratio on diastolic function parameters in subjects with dilated cardiomyopathy (DCM) or hypertrophic cardiomyopathy (HCM) and healthy controls. Methods and results We performed echocardiography in 44 healthy controls, 35 HCM subjects, 29 DCM subjects with narrow QRS complex (DCM-n), and 27 DCM subjects with wide QRS complex (DCM-w). Mitral annulus velocity (Ea) and transmitral E-wave velocity were used to estimate time constant of isovolumic pressure decay (τ). LV flow propagation velocity (Vp) and early intraventricular pressure gradient (IVPG) were derived from colour M-mode of LV inflow. We calculated LV twist and peak untwisting rate (UntwR) by speckle tracking. Mean LV volume/mass ratio was 0.34 ± 0.09 mL/g in healthy controls, 0.15 ± 0.06 mL/g in HCM, 0.6 ± 0.2 mL/g in DCM-n, and 0.8 ± 0.3 mL/g in DCM-w patients (P < 0.001 for all groups). Resting LV ejection fractions were 63 ± 7, 64 ± 8, 31 ± 8, and 26 ± 8%, respectively (P < 0.01 vs. controls for DCM groups). In a multivariate analysis, LV volume/mass ratio remained a strong independent predictor of Vp (P < 0.001), IVPG (P = 0.009), and UntwR (P < 0.001) but not for Ea (P = 0.25). Conclusion LV volume/mass ratio had influences on diastolic function parameters independent of intrinsic diastolic function and filling pressures. It should be considered when assessing patients suspected of LV diastolic dysfunction.