Glycogen storage disease type II has a broad continuous clinical spectrum in terms of onset, involvement of organs and life expectancy. Infantile onset is the most severe form, presenting with prominent cardiomyopathy, hypotonia, hepatomegaly and death before 12 months of life. Late onset form has onset at any age, lack of severe (or absence of) cardiac involvement, progressive skeletal muscle dysfunction and less dismal short-term prognosis. In addition to muscle and heart involvement, other tissues are affected liver, spleen, endothelium, lung, brain, anterior horns, peripheral nerves. In fact some patients with infantile form have hearing loss, abnormal brain myelination and central fever and some adult patients show aneurysms of brain arteries due to accumulation of glycogen in vessels. As for other treatable lysosomal diseases, the advent of enzyme replacement therapy will change the natural history of this disease and also will increase our knowledge concerning clinical heterogeneity.