A 26-day-old male infant who had been fussy and feeding poorly for a period of several hours died suddenly despite efforts at resuscitation. Postmortem examination revealed eosinophilic endomyocarditis unassociated with disease in other organs. The etiology remained unexplained after review of the medical and family histories and circumstances of death, extensive light and immunofluorescence microscopies, and microbiological, metabolic, and toxicologic testing. This appears to be the youngest reported case of eosinophilic endomyocarditis, a disorder that typically occurs in middle-age males and is often associated with benign or malignant hypereosinophilic conditions. The medical literature is reviewed.