Aims Right ventricular apex (RVA) pacing may have deleterious effects on left ventricular (LV) systolic function, but its impact on LV diastolic function has not been explored. Methods and results Ninety-seven patients with sinus node dysfunction and ejection fraction (EF) >= 50% with permanent RVA pacing were randomly programmed to V-sense and V-pace modes and examined by echocardiography. Tissue Doppler imaging was employed to assess myocardial systolic velocity (S') and early diastolic velocity (E') at the mitral annulus. Systolic dyssynchrony was assessed using 12 LV segmental model (Ts-SD). Switching from V-sense to V-pace resulted in the worsening of both diastolic and systolic functions as shown by the decreased EF, reduced mean E' and S' velocities, as well as increase in LV volume and Ts-SD (all P < 0.001). Reduction of mean E' and S' of >= 1 cm/s occurred in 35 (36%) and 45 (46%) patients, respectively. In pre-defined subgroup analysis, only patients with pre-existing LV diastolic dysfunction had a significant reduction of mean E' and S' (both P < 0.001) even after age adjustment. Multivariate logistic regression analysis showed that independent factors for the reduction of mean E' >= 1 cm/s or mean S' >= 1 cm/s at V-pace were pre-existing LV diastolic dysfunction [ odds ratio (OR): 4.735, P = 0.007 for E'; OR: 3.307, P = 0.022 for S'] and systolic dyssynchrony at V-pace (OR: 5.459, P = 0.007 for E'; OR: 2.725, P = 0.035 for S'). Conclusion In patients with preserved EF, RVA pacing is associated with the deterioration of both LV diastolic and systolic functions, which is particularly obvious in those with pre-existing LV diastolic dysfunction and V-pace-induced systolic dyssynchrony.