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During a previous investigation and during routine clinical Doppler echocardiography velocities distal to normal valves were found to be increased in patients with congenital heart disease. To investigate this observation the velocity proximal and distal to cardiac valves was recorded in 56 patients with congenital heart disease. No detectable pressure gradient had been found across these valves at catheterisation and no velocity gradient greater than 20 cm/s was found across them by range gated Doppler echocardiography. In 82% of the patients, however, the blood velocity across one or more valves exceeded normal limits. Increased velocities were found at the tricuspid inflow (55%), pulmonary artery (38%), mitral inflow (34%), and ascending aorta (11%). Factors predisposing to the presence of increased velocities in these areas were increased flow through the valve and decreased compliance of the receiving chamber. This study shows the necessity for recording velocity both proximal and distal to a valve before a gradient is calculated according to the modified Bernouilli equation.