The pressure-velocity relationship across the normal mitral valve is approximated by the Bernoulli equation P = 1/2 v² + M · dv/dt, where P is the atrioventricular pressure difference,  is blood density, v is transmitral flow velocity, and M is mitral inertance. Although M is indispensable in assessing transvalvular pressure differences from transmitral flow, this term is poorly understood. We measured intraoperative high-fidelity left atrial and ventricular pressures and simultaneous transmitral flow velocities by using transesophageal echocardiography in 100 beats (8 patients). We computed mean mitral inertance () by  = (P1/2 · v²)dt/(dv/dt)dt and we assessed the effect of the inertial term on the transmitral pressure-flow relation. ranged from 1.03 to 5.96 g/cm² (mean = 3.82 ± 1.22 g/cm²). P calculated from the simplified Bernoulli equation (P = 1/2 · v²) lagged behind (44 ± 11 ms) and underestimated the actual peak pressures (2.3 ± 1.1 mmHg). correlated with left ventricular systolic pressure (r = 0.68, P < 0.0001) and transmitral pressure gradients (r = 0.65, P < 0.0001). Because mitral inertance causes the velocity to lag significantly behind the actual pressure gradient, it needs to be considered when assessing diastolic filling and the pressure difference across normal mitral valves.