To evaluate the correlation of cardiothoracic ratios (CTR) on chest radiography with the sizes of the four cardiac chambers, we studied 41 normal persons and 77 cardiac patients using two-dimensional echocardiography (2DE). The transverse cardiac diameter and the CTR were ascertained from their chest radiographs. Using 2DE, each cardiac chamber was evaluated three-dimensionally, by measuring the short and long diameters of each left (LV) and right ventricle (RV), the diameter of the RV outflow, the anteroposterior, transverse, and long diameters of each left (LA) and right atrium (RA). These values were corrected using the square root of the body surface area. The correlation coefficient for the CTR was greatest in the long diameter of the LA (r = 0.84), and was greater in the atrial diameters (r greater than or equal to 0.71) than in the other diameters except for the transverse diameter of the RA (r less than or equal to 0.51). The contribution of each cardiac chamber to the transverse cardiac diameter was evaluated in 34 patients with cardiac enlargement by a method using the null hypothesis, which we originally designed. Consequently, the contribution of the LV to the CTR was definitely proven in two patients, that of RV in two, that of LA in 15 and that of RA in nine. These results indicated that the CTR is strongly influenced by enlargement of the LA.