Coffin-Lowry syndrome is an X-linked recessive syndrome of mental retardation, characteristic facies and skeletal anomalies. In one patient with the syndrome, we observed early recurrent episodes of congestive heart failure with intercurrent normalization and the late development of mitral insufficiency due to annular dilation and congenital abnormalities of the valve apparatus. This unusual course of cardiac involvement, the non-adaptation of the left ventricular contractility to the aggravation of the mitral insufficiency and the postoperative persistence of the ventricular dysfunction, underline the possible role of an associated primary myocardial disease. This clinical observation demonstrates clearly that a mitral valve malformation can occur in patients with the syndrome, but also the role of a dilated cardiomyopathy, which can be secondary to the mitral regurgitation, but is more likely a myocardial disorder occurring as part of the syndrome.