The aim of the study was to evaluate the accuracy of echocardiographic quantification of mitral valve opening area in severe mitral stenosis. 31 consecutive patients with severe mitral stenosis were studied with two-dimensional echocardiography before they had complete resection of the mitral valve. The valves were examined for calcifications by x-ray. Each specimen was tensionlessly suspended in a glass cylinder, with 10 to 15 l of warm water (37 degrees C) running through it until maximal opening of the valve. Then the valvular orifice was photographed for planimetry. Now the echocardiographic results were checked again to analyse the errors of the initial assessment. In 6 out of 31 patients the size of the valvular opening area could not be assessed echocardiographically due to poor echo quality. The mean mitral opening area of the specimens was 0.92 +/- 0.32 cm2. With 1.27 +/- 0.52 cm2, the results achieved by echocardiography reached a correlation of only r = 0.44. In 9 out of 25 patients the area was assessed precisely in terms of size and anatomy. The difference between the values calculated from the specimens and echocardiograms was below 0.5 cm2 in 19 out of 25 (76%) patients and below 1 cm2 in another 4 (16%) patients. A larger difference in two patients was due to incorrect beam direction. Otherwise, false results in 10 out of 25 patients were caused by multiple inner echoes and in 2 out of 25 patients by bright reflections due to calcifications. Although the echocardiographically assessed mitral valve opening area does not correlate with the real opening area, it is possible to distinguish in most patients between severe and mild stenosis. Furthermore the valvular opening area can be exactly determined up to 0.5 cm2 in 90 percent of patients, provided that the echo beam is correctly positioned.