Introduction: The study tested the feasibility of differentiating radiofrequency ablation lesions from normal myocardium and quantifying their dimensions by myocardial contrast echocardiography (MCE). Methods and Results: In 11 normal dogs, we created 14 focal and 4 linear lesions at different left ventricular sites. MCE was performed both before and after ablation by using an intracardiac echocardiography catheter (9 MHz) and infusing contrast microbubbles through the left coronary artery. An independent observer examined the lesion pathology. We found that intracardiac echocardiography alone could not delineate lesion dimensions. However, after ablation, MCE localized the lesions as well-defined, low-contrast areas within the normally opacified myocardium. Lesion dimensions by MCE immediately after ablation and 30 minutes later were similar. In 12 focal lesions, the average maximum depth (5.55 ± 1.38 mm) and average maximum diameter (10.38 ±2.09 mm) by MCE were in excellent agreement with the pathologic depth (5.20 ± 1.45 mm) and diameter (10.61 ± 1.67 mm). Two focal lesions could not be detected by MCE and later were found to be superficial. Three-dimensional MCE correctly reconstructed the extent and shape of linear lesions compared to pathology (length: 18.7 ± 5.7 vs 18.5 ± 5.6 mm; maximum longitudinal cross-sectional area: 81.2 ± 9.6 vs 76.0 ± 10.3 mm2). Conclusion: MCE accurately localized and quantified radiofrequency ablation lesions in the normal left ventricle. This new application of MCE may advance ablation for managing ventricular arrhythmias that involve intramural or epicardial regions by providing instantaneous anatomic feedback on the effects of ablation during catheterization.