Routine echocardiographic examinations were performed on 53 individuals with muscular dystrophy (29 with myotonic muscular dystrophy [MMD] and 24 with Duchenne's dystrophy [DD]). Five of 29 (17%) with MMD and six of 24 (25%) with DD had typical mitral valve prolapse evidenced by echocardiography. All patients with DD and mitral valve prolapse had severe thoracic skeletal deformities, while none with MMD and mitral valve prolapse had such abnormalities. Other echocardiographic parameters analyzed included the fractional shortening of the left ventricle (%ΔS), excursions of the left ventricular posterior wall (LVPW) and the ventricular septum, pre-ejection period/ left ventricular ejection time ratios (PEP/ LVET), and thickness of the LVPW and ventricular septum. Abnormal prolongation of the PEP/LVET ratio and increased thicknesses of the LVPW and ventricular septum were noted in some individuals in the MMD group. Decreased %ΔS with diminished excursion and diastolic velocity of the LVPW were noted in some with DD. © 1980, American Medical Association. All rights reserved.