Fatal myocarditis is a rare complication in immunosuppressed children. Recent reports have linked human herpesvirus 6 (HHV-6) infection, typically a benign infection in childhood, with myocarditis. HHV-6 can reactivate during periods of immunosuppression. Here, we report 2 cases in which children were immunosuppressed, one for treatment of Evans syndrome and the other post hematopoietic stem cell transplantation, who developed rapid and fatal HHV-6-associated myocarditis. These cases suggest that HHV-6 infection should be considered as an etiology of myocarditis in immunosuppressed patients regardless of correlating blood levels. Early treatment of HHV-6 in patients with myocarditis could improve morbidity and mortality.