The supine anterior-posterior(AP) view of the chest with a F.F.D. of 40 inches is usually obtained in serious patient who is unable to stand upright. It is very important clinically to evaluate cardiac size in such seriously ill cases. but is often difficult to determine whether the heart is actually enlarged or not on the supine AP view because no standard criteria has not been established yet unlike on the PA view. Present study was carried out to determine the normal values of cardiac transverse diameter and cardiothroacic ration(CTR) on the supine AP view, and they were correlated to the valus of the PA view. Present study was carried out to determine the normal vaues of cardiac transverse diameter and cardiothoracic ratio (CTR) on the supine AP view, and they were correlated to the values of the PA view. The materials consist of 165 normal Korean adults. Both erect PA view with a F.F.D of 72 inches and supine AP view with a F.F.Dof 40 inches were taken for each cases. The result obtained were as follows. 1. The internal thracic diatmeter was 30.5$pm$1.8cm in male and 27.7$pm$1.5cm in female on PA view. and 32.4$pm$2.0cm and 29.9$pm$1.6cm respectively in AP view. 2. The cardiac transverse diameter was 12.9$pm$1.2cm in male and 12.3$pm$1.0cm in female on PA view, and 15.8$pm$1.3cm and 14.6$pm$1.0cm respectively on AP view. 3. The cardiac transverse diameters were correlated between PA view (X) and AP view (Y) with an equation of Y=4.7 +0.865$ imes$in male, and Y=4.4 +0.863X in female. 4. The CTR was 0.430$pm$0.041 in male and 0.444$pm$0.035 in female on PA view, and 0.490$pm$0.036 and 0.497 $pm$0.038 respectively on AP view.