Cardiovascular studies were performed on 22 patients with mucopolysaccharidosis ascertained from an epidemiological study on the mucopolysaccharidoses in Northern Ireland. None of the patients had attended a cardiologist before the study. The main echocardiographical findings were thickening of the interventricular septum and left ventricular posterior wall in the absence of ECG evidence of ventricular hypertrophy. Moreover, reduced QRS voltages were present in the majority of the patients (77%) and some had reduced shortening fraction (33%). These findings suggest an infiltrative cardiomyopathy owing to mucopolysaccharide deposition as a cause of the cardiac thickening rather than true ventricular hypertrophy. Thickening of the mitral valve was present in one case and thickening of the aortic valve in two cases. Involvement of the other heart valves was minimal and aortic valve disease was not found in any of the cases of Morquio's disease type A. In conclusion, the clinical, ECG, and chest x ray findings and echocardiographical evidence for valvular involvement were significantly less than in other studies. Hence, the incidence of clinically significant cardiovascular disease in patients with mucopolysaccharidosis has probably been overestimated.