Reactive eosinophilia is associated with inflammatory bowel disease, but its association with eosinophilic myocarditis is rare. We report a case of a 42-year-old man who presented with hypovolemic shock secondary to diarrhea and recently diagnosed nonischemic cardiomyopathy (left ventricular ejection fraction, 0.29). Laboratory evaluation revealed marked peripheral eosinophilia. Cardiac magnetic resonance imaging showed evidence of subacute-to-chronic myocarditis, and endomyocardial biopsy results were consistent with eosinophilic myocarditis. Colonic biopsy specimens revealed ulcerative colitis and no eosinophils. Hematologic evaluation was negative for an alternative cause of eosinophilia. The patient was given corticosteroids; his diarrhea resolved, but there was no short-term improvement in his ejection fraction, so an implantable cardioverter-defibrillator was placed. Follow-up at one year showed that the patient's left ventricular ejection fraction had improved to 0.42.