Immunisation may induce myocardial complications. In this pilot study clinical, electrocardiographic, chemical and immunological findings have been studied during a six weeks' follow-up after routine immunisation (mumps, polio, tetanus, smallpox, diphtheria and type A meningococcal disease) among 234 Finnish conscripts at the beginning of their military service. Serial pattern of ECG changes suggestive of myocarditis was recorded in eight of the 234 conscripts one to two weeks after vaccination against smallpox and diphtheria. Changes were mainly minor ST segment elevations and T wave inversions and usually they disappeared in a few weeks. The ECG positives more often had a history of atopy, and their mean body temperatures and heart rates after the vaccinations were higher than among the other subjects (p less than 0.01). However, clinical myocarditis was never noted, nor were immunological or enzymological changes different among the ECG positives. Thus in 3% of the study population, evidence of postvaccinal myocarditis was noted, based on serial ECG patterns, but without any other evidence of cardiac disease.