The authors describe the clinical picture, ECG, vectorcardiographic, and serum enzyme assays in one case of acute myocardial infarction occurring after a severe physical exertion in a child aged 10 with progressive muscular dystrophy of the Duchenne type. The picture differs considerably from the electrical aspects of "pseudonecrosis" that interstitial fibrosis of some portions of the left ventricle may produce in this myopathy.