Abstract
A 21-year-old woman with a history of atopy, peripheral eosinophilia, Wolf-Parkinson-White syndrome, and 5 episodes of myocarditis was diagnosed with eosinophilic myocarditis. Despite adequate immunosuppressive treatment and resolution of the myocarditis episode, the patient developed dilated cardiomyopathy and presented with worsening of her functional class. Finally, genetic testing unveiled an additional diagnosis: Danon disease. (Level of Difficulty: Advanced.).