The etiology of mild myocarditis, diagnosed on the basis of serial ECG changes during an acute infection, was studied in 126 consecutive conscripts. A fourfold rise in the antibody titers in the paired serum samples was required for a positive etiologic diagnosis. An etiologic diagnosis was made probable in 47% of the patients. Adenovirus was incriminated in 19 patients, vaccinia in 12, influenza A in eight, beta-hemolytic Streptococcus in six, mononucleosis in five and Mycoplasma in three. Chlamydia, influenza B and Coxsackie B4 were each found in two patients; parainfluenza, mumps and adult Still's disease were each found in one patient. The incidence of vaccinia myocarditis was 1/10000 smallpox vaccinations. Clear-cut myopericarditis was usually noted during vaccinia, mononucleosis, Mycoplasma, Chlamydia and Coxsackie B4 infections. Adenovirus and influenza A myocarditis was most often subclinical, being mostly detected only because of ECG screening of patients without cardiac symptoms. Frequent recent ventricular extrasystoles were most often triggered by a beta-hemolytic Streptococcus infection. The etiology of infectious myocarditis seems to reflect the overall profile of viruses and other infective agents in the study population at that particular time. Cardiotrophic viruses such as Coxsackie B only rarely cause myocarditis outside epidemics.