Diastolic suction has been demonstrated experimentally as a ventricular negative pressure when the ventricle is allowed to relax completely in the absence of filling, but it has not been extensively studied in the in vivo human heart. In balloon mitral valvuloplasty with a single balloon, the mitral orifice is occluded and inflow is considered to be completely obstructed during a balloon inflation. To demonstrate diastolic suction in the human ventricle, we measured left ventricular pressure during valvuloplasty with a high-fidelity catheter tip manometer in 17 patients. Left ventricular pressure fell below zero during a balloon inflation in all patients (-2 to -12 mm Hg). The peak negative diastolic pressure showed significant correlations with end-systolic volume index (r = 0.53, p = 0.03) and with the ejection fraction (r = 0.80, p = 0.0001). Thus diastolic suction was demonstrated in the human beating heart, and the sucking effect was potent in the heart with small end-systolic volume and high-ejection fraction.