BACKGROUND: Despite more accurate methods of cardiac imaging, plain chest X-ray (CXR)still is the first imaging method in people with suspected heart diseases. The most importantapplication of CXR in cardiac patients is the estimation of cardiac size. There are two mainmethods for cardiac estimation in CXR; transverse diameter (TD) and calculation of cardiacwidth ratio to thoracic cage or cardiothoracic ratio (CTR). Echocardiography is a standard andmore accurate method for cardiac size measurement; however, it requires more time and iscostly,and is not as accessibleas CXR.METHODS: This study aimed to determine the sensitivity and specificity of CXR to diagnoseenlarged heart (cardiomegaly) in comparison with echocardiography. In this study, 327 adultsthat referred for echocardiography to Hajar Hospital(Shahrekord, Iran) during summer 2001were recruited. Their CXR findings were compared to echocardiography.RESULTS: Mean age of the patients was 53.6 years and 46% were male. Considering the CTR ≤50% of thoracic cage widthand TD ≤ 16 cm as normal,CTR resulted in 28.2% false-positiveand9.2% false-negative classification for cardiomegaly.The corresponding figures for TD were8.8%and 58.5%, respectively.CONCLUSION: In order to determine cardiomegaly, CTR had a higher sensitivity incomparison with TD; however, the specificity of TD was higher than CXR.Keywords: Echocardiography, Plain Chest X-Ray, Cardiac Size.