Magnetic resonance imaging has become an important diagnostic tool in the differential diagnosis of lesions for evaluation of cardiovascular disorders. In magnetic resonance tagging (MRt), tissue elements are magnetically labeled so that their positions can be tracked as a function of time. Thus, MRt evaluates heart wall motion both qualitatively and quantitatively. We present herein the case of a 12-year-old boy who had chest pain, dyspnea on effort and murmur. On cardiac computed tomography, there was focal thickening of the left ventricular posterior wall, similar to a mass. MRt indicated active displacement and deformation of the tags at the level of the hypertrophic myocardium during systole, as with normal myocardium. Thus, the tagged images supported the diagnosis of focal hypertrophic cardiomyopathy (HCM). In view of these results, MRt should be considered as a useful technique for differentiating between a mass-like focal lesion such as neoplasm and HCM.