Two cases of cardiac glycogen storage disease type II are described: the first one, male aged 3 months, presented with generalized muscular hypotonia and decreased deep tendon reflexes; a 2/6 systolic murmur was audible at the left sternal border; chest X-ray and ECG were consistent with left ventricular hypertrophy; an echocardiogram disclosed an impressive and diffuse cardiac hypertrophy; the pump function appeared preserved and the estimated ejection fraction was about 70%. Pulsed wave Doppler demonstrated a normal envelope of mitral flow with E/A ratio = 1.27 in averaged 20 beats. The patient died suddenly at 6 months of age. The second patient was a female 4 months old with generalized muscular hypotonia. ECG and chest X-ray were consistent with left ventricular hypertrophy; 2D echocardiogram showed diffuse hypertrophy with estimated ejection fraction of 68% and an almost normal aspect of the mitral flow curve, with E/A ratio of 1.18. This child died at 13 months of age of cardiopulmonary insufficiency. In both cases the diagnosis was made by muscular biopsy and biochemical tests (alpha 1-4 glucosidase deficiency). We stress the fact that, despite the severe and diffuse hypertrophy, the pump function and the ventricular filling did not seem compromised.