Echocardiography was performed in 16 children undergoing bone marrow transplantation (BMT) for mucopolysaccharidoses. Cardiac involvement before BMT was detected in seven (44%). One year after BMT (11 patients/five with cardiac involvement), left ventricular restriction resolved in 2/3 patients and hypertrophy in one. In the remainder, at mean follow up of 2.5 years, no progression of preexisting or development of new cardiac involvement was noted. It is concluded that in a significant proportion of patients with mucopolysaccharidoses, cardiac involvement improved after BMT.