A 4-year-old child is described who suffered an unsuspected myocardial contusion which led to the formation of a ventricular aneurysm 2 months after an automobile accident. Electrocardiography, thallium scanning, myocardial enzyme assays, two-dimensional echocardiography and, when indicated, cardiac catheterization, may aid in the early diagnosis of cardiac contusions. In the patient presented, resection of the aneurysm 6 months postinjury was followed by elimination of almost all the mitral regurgitation originally present and good cardiovascular function.