It is generally believed that patients with dilated cardiomyopathy have a large cardiac silhouette on chest roentgenography. Contrary to this general belief, we have recently examined several patients with a dilated left ventricle (LV) on echocardiography but in whom the cardiothoracic ratio (CTR) was within normal limits. To investigate this apparent discrepancy, we evaluated the relationship between LV dimensions, measured on M-mode echocardiography, and CTR in 49 patients with dilated cardiomyopathy. Among these patients, 11 (22%) had a CTR less than 50% and 38 (78%) had a CTR greater than 50%. The spatial orientation (cardiac rotation) of the LV within the thorax was evaluated by magnetic resonance imaging (MRI) in 5 patients with a CTR less than 50% and in 7 patients with a CTR greater than 50%, in comparison with 7 normal controls. In each of these patients, cardiac rotation was assessed from both a transverse and a frontal MRI section. In both groups, LV end-diastolic dimension was greater than 5 cm. Transverse cardiac rotation was 32 +/- 8 degrees in patients with a CTR less than 50%. This was significantly lower than in the 7 normal controls (43 +/- 7 degrees) (p less than 0.05). In patients with a CTR greater than 50%, however, transverse cardiac rotation (55 +/- 5 degrees) was significantly greater than in normal controls (p less than 0.01). No differences in frontal cardiac rotation was observed between the 2 groups. These data indicate that a normal cardiac silhouette in patients with dilated cardiomyopathy can be explained on the basis of a counterclockwise transverse rotation of the heart within the thorax, and it cannot always rule out the dilatation of the LV.