The significance of minor myocardial inflammatory infiltrates and viral detection in SIDS is controversial. We retrospectively compared the demographic profiles, myocardial inflammation, cardiomyocyte necrosis, and myocardial virus detection in infants who died of SIDS in a safe sleep environment, accidental suffocation, or myocarditis. Formalin-fixed, paraffin-embedded myocardial sections were semiquantitatively assessed for CD3 lymphocytes and CD68 macrophages using immunohistochemistry and for cardiomyocyte cell death in H&E-stained sections. Enteroviruses and adenoviruses were searched for using PCR technology. The means of lymphocytes, macrophages, and necrotic cardiomyocytes were not statistically different in SIDS and suffocation cases. Enterovirus, not otherwise specified, was detected in one suffocation case and was the only virus detected in the three groups. Very mild myocardial lymphocyte and macrophage infiltration and scattered necrotic cardiomyocytes in SIDS are not pathologic, but may occur after the developing heart is exposed to environmental pathogens, including viruses.