Background: Diastolic heart failure (DHF) is a leading cause of mortality and morbidity and represents about 50% of all HF cases. Yet, DHF remains poorly understood and few therapeutic advancements have been made for its management. Recent evidence suggests that DHF has comparable systolic dyssynchrony to that of HF with a reduced ejection fraction (EF). In HF with reduced EF, bi-ventricular (Bi-V) pacing has emerged as a highly effective therapy. This study examines the acute effects of Bi-V pacing in DHF with significant ventricular systolic dyssynchrony. Methods: Twelve DHF patients with EF >50% and echocardiographic evidence of mechanical dyssynchrony were studied while undergoing cardiac catheterization studies. Patients where instrumented with temporary pacing catheters in the RA, LV and RV. Systolic dyssynchrony (using Tissue Doppler) and ECG were measured at baseline and during a brief period of Bi-V pacing (5 min). Patients were paced in VDD mode with AV timing selected to optimize transmitral flow and with simultaneous RV-LV timing. The dyssynchrony metrics assessed were: Basal and 12 myocardial segment septal to free wall delay (basal S-FW, 12seg S-FW); dispersion of time to peak systolic velocity in 12 segments (Ts Disp); 12 segment standard deviation of time to peak systolic and diastolic velocity (12seg Ts-SD and Te-SD). Results: Consistent with general DHF demographics reported historically, the patients in this study were 69±10 years old with a female majority (70%), hypertensive (100%) with a mean NYHA functional class of 2.6±0.5, and a high body mass index (27±4Kg/M2). Patients also had a mean EF of 68±15% and a predominantly narrow QRS (91±11ms). Cardiac dimensions were not dilated (LVEDD: 43±7 mm and LVESD: 26±4 mm). In an anesthetized state, Bi-V pacing significantly (p<0.02) improved all systolic and diastolic dyssynchrony measures compared to baseline (basal S-FW: 93±47 vs 57±53ms; 12seg S-FW: 92±48 vs. 60±49ms; Ts Disp: 108±52 vs. 78±45ms; 12seg Ts-SD: 55±9 vs. 42±13ms; 12seg Te-SD: 28±12 vs. 20±7ms), despite a significant increase in mean QRS width (91±11 vs. 129±18ms, p<0.01). Conclusions: These data suggest that acute Bi-V pacing may improve systolic and diastolic ventricular dyssynchrony in a DHF population with systolic dyssynchrony, despite a preserved EF and narrow QRS. Whether these acute improvements in cardiac performance are matched by a chronic therapeutic benefit with Bi-V pacing in this population will require further study.