Description: Background— Normal coronary blood flow is principally determined by a backward-traveling decompression (suction) wave in diastole. Dyssynchronous chronic heart failure may attenuate suction, because regional relaxation and contraction overlap in timing. We hypothesized that biventricular pacing, by restoring left ventricular (LV) synchronization and improving LV relaxation, might increase this suction wave, improving coronary flow. Method and Results— Ten patients with chronic heart failure (9 males; age 65±12; ejection fraction 26±7%) with left bundle-branch block (LBBB; QRS duration 174±18 ms) were atriobiventricularly paced at 100 bpm. LV pressure was measured and wave intensity calculated from invasive coronary flow velocity and pressure, with native conduction (LBBB) and during biventricular pacing at atrioventricular (AV) delays of 40 ms, 120 ms, and separately preidentified hemodynamically optimal AV delay. In comparison with LBBB, biventricular pacing at separately preidentified hemodynamically optimal AV delay (BiV-Opt) enhanced coronary flow velocity time integral by 15% (7%–25%) ( P =0.007), LV dP/dt max by 15% (10%–21%) ( P =0.005), and neg dP/dt max by 17% (9%–22%) ( P =0.005). The cumulative intensity of the diastolic backward decompression (suction) wave increased by 26% (18%–54%) ( P =0.005). The majority of the increase in coronary flow velocity time integral occurred in diastole (69% [41%–84% ]; P =0.047). The systolic compression waves also increased: forward by 36% (6%–49%) ( P =0.022) and backward by 38% (20%–55%) ( P =0.022). Biventricular pacing at AV delays of 120 ms generated a smaller LV dP/dt max (by 12% [5%–23% ], P =0.013) and neg dP/dt max (by 15% [8%–40% ]; P =0.009) increase than BiV-Opt, against LBBB as reference; BiV-Opt and biventricular pacing at AV delays of 120 ms were not significantly different in coronary flow velocity time integral or waves. Biventricular pacing at AV delays of 40 ms was no different from LBBB. Conclusions— When biventricular pacing improves LV contraction and relaxation, it increases coronary blood flow velocity, predominantly by increasing the dominant diastolic backward decompression (suction) wave.