Newer pacemakers designed for CRT have the possibility to program a delay between paced ventricles (VV delay). There is some evidence that VV delay optimization in biventricular pacing (BiVp) may bring an additional hemodynamic benefit of CRT. Most of the CRT studies conducted so far were performed with no VV delay programming possibility. of the study was to compare acute hemodynamic effects of BiV pacing using different times of right ventricle (RV) after left ventricle (LV) delay. The study group consisted of 71 patients (59M, 12F; mean age 67,2 y.) with permanently implanted BiVp system with the possibility to set the VV delay. RV lead: 49 pts - RVA, 22 pts RVOT. Measurements were performed by means of impedance cardiography (BioZ.com). Stroke Volume (SV), Velocity index (VI), Acceleration Index (ACI) as well Ejection Time (LVET) and Preejection period (PEP) were determined. The measurements in 3 min periods were collected and averaged, after the adaptation periods of 3 min throughout different VV delay pacing modes (0/4ms, 15/16ms, 30/32ms, 50/52ms and 72/75ms, depending on the pacemaker). Mean SV values increased as VV delay was prolonged, from the lowest at 0ms VV delay to the highest at 75ms VV delay (62,4; 62,7; 62,9; 64,9; 65,6 ml respectively; ANOVA p<0,001). Different responses were observed in individual patients, however 75ms delay was the best setting in 36,6% and 0ms the worst in 31,0% of pts. The mean intrapatient difference between BiVp with no delay and the best one was 6,9ml (>10%) (p<0,001). Sequential BiVp compared to BiVp with no VV delay brings the additional hemodynamic improvement in acute setting. In the majority of patients the VV delay of 75 ms is the most beneficial and no delay is the least beneficial. Because of inter-patient differences, the VV delay should be optimized in every case of BiVp.