Heart failure is a major cause of death in boys with Duchenne muscular dystrophy. In order to determine if the cardiac function of the mdx mouse is similarly disturbed, we performed murine echocardiograms and left heart catheterization studies, along with morphometric analysis of cardiac fibrosis. Serial echocardiograms in mdx mice revealed the evolution from normal cardiac function in young mice to a dilated cardiomyopathy in adult mice. Very old mdx mice exhibited a widespread but patchy increase in ventricular wall fibrosis. These results show that the mdx cardiac function is more impaired than was previously thought and shares important clinical features with the cardiomyopathy of Duchenne muscular dystrophy.