An in vitro study was conducted, using a stenotic model to demonstrate that different pressure gradients can be produced by the same degree of valvular stenosis. This model is comprised of two cylindric chambers with a diaphragm in the center which had a small central hole. An injector was connected to one end of the prestenotic chamber to produce a steady pulsed flow. A rubber tube was connected to the other end of the poststenotic chamber and led upward to a large reservoir which provided a constant afterload pressure. Two pressure transducers were attached to the two connecting tubes, both linked with two pigtail catheters which were accommodated in the chambers just before and after the stenotic diaphragm. Two sets of injection volumes (20 and 30 ml) and multiple injections with different flow rates (5, 10, 15,...49 ml/s) were administered and resulting pressures measured by the two transducers were recorded. Results showed that different pressure gradients could be produced using the same injection volume, the same afterload, and the same degree of stenosis. The greater the flow rate, the higher the pressure gradient. Good correlation existed between the pressure gradient and the injection flow rate (r = 0.95 and 0.97 for the study groups receiving 20 and 30 ml injection volumes, respectively; p < 0.001 in all comparisons). Thus, a higher pressure gradient may not necessarily indicate a severe degree of valvular stenosis. Evaluation of a stenotic lesion should not be made from the degree of pressure gradient alone--other hemodynamic conditions should also be taken into account.