Adenovirus type 21 was isolated from the lungs of an 11-month-old infant who died of a nonsuppurative myocytolytic myocarditis with lesions present in both left and right ventricular myocardium, and a focal interstitial pneumonitis. Virus particles were not seen in heart muscle under the electron microscope. Other viral or bacterial pathogens were not isolated. Thus, it appears that adenovirus type 21 should be included in the group of viruses which, on occasion, are associated with fatal cardiorespiratory disease.