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The determination of radiographic cardiac size as an estimation of the state of cardiac function is one of the more common correlations made. Despite the widespread use of these measurements, the correlation between cardiac function and radiographic appearance, and the validity of clinical judgments based on this correlation, has not been fully determined. Similarly, the increment in left ventricular chamber size necessary to produce a change in the plain film appearance of the left ventricle has not been defined.The results of a two observer, blind, retrospective analysis of plain film radiographs of the chest, and related quantitative left ventricular angiograms, and left ventricular pressure studies performed on 256 patients is presented. The sensitivity, speficity, predictive value and accuracy of six previously described plain film measurements of left ventricular size together with determinations of the extent of left ventricular volume change necessary to produce a perceptible change in the plain film radiographic appearance of the left ventricle are reported. The performance of each of these measurements proved to be disappointing. A sensitivity of 75% was not possible using any of the methods unless the left ventricular chamber volume was more than 66% above the upper limit of normal.