Cardiac involvement is well recognised in most forms of the mucopolysaccharidoses but there is poor documentation of abnormalities specific to Morquio's syndrome (type IV mucopolysaccharidosis). Ten patients with the classic form or type A Morquio's syndrome with a median age of 12.5 years underwent echocardiographic assessment. Abnormalities were detected in six (60%) cases with mitral valve involvement in five patients and aortic valve disease in four. One patient had severe mitral leaflet thickening to the point of mitral stenosis. Two patients had evidence for myocardial involvement by way of echocardiographic ventricular hypertrophy. The cardiac lesions were haemodynamically mild. A cardiac murmur was audible in only three of the six cases. It is concluded that there is a high prevalence of silent cardiac abnormalities in patients with Morquio's syndrome with predominantly left sided valve involvement. Echocardiography should be part of the assessment of these patients and bacterial endocarditis prophylaxis should be advised for those with cardiac abnormalities.