We studied three patients with influenza virus-associated fulminant myocarditis; one was infected by type B and the others by type A influenza virus. In one patient, dissemination of type A (H1N1) virus to the myocardium was demonstrated, and viremia complicated the clinical course despite the use of oral amantadine HCl and ribavirin aerosol. All patients were treated with iv ribavirin, two initially and the third after viremia was detected during hyperacute rejection of a cardiac transplant. No significant adverse effects could be directly attributed to therapy, and viral shedding abruptly terminated coincident with its use; however, both patients treated shortly after onset of myocarditis died. The third required support by an artificial heart, and died 8 mo later. Immunotyping of myocardial tissues in two cases revealed an initial predominance of T helper cells. Serial endomyocardial biopsies available from one of these demonstrated a subsequent marked decrease in the T helper cell population as inflammation and necrosis subsided during and following therapy.