Echocardiography is the technique of choice for the assessment of replacement aortic valves. Hemodynamic function depends on the design and size of the valve. This review summarizes the published information available to the end of 2005. The most obstructive valve was the caged-ball, followed by the stented porcine and single tilting-disc valves. The stented bovine pericardial valves were slightly less obstructive than these, and similar to the intra-annular bileaflet mechanical valves. Stentless valves appeared slightly less obstructive still, and similar to reduced-cuff mechanical bileaflet valves. Homografts were the least obstructive. Approximate guide thresholds suggesting obstruction were derived.