Type II mucopolysaccharidosis (Hunter's disease) is a hereditary condition due to a deficit of a lysosome specific hydrolase (iduronate sulfatase) inducing an accumulation of dermatane-sulphate and heparane-sulphate in certain organs. Cardiac involvement is constant in this disease and manifests itself essentially by aortic valve stenosis and insufficiency and/or mitral insufficiency which is progressive, irreversible and life-threatening. Two brothers of Portuguese nationality suffering from a so-called slight form of this disease had classical mucopolysaccharide infiltration of their aortic valves. The elder brother, aged 11, had severe aortic insufficiency associated with mild stenosis requiring treatment with vasodilator drugs. The younger, aged 8, had asymptomatic mild aortic regurgitation. Curiously, mitral valve prolapse with regurgitation was present in both children. The association of mitral valve prolapse and type II mucopolysaccharidosis, without other typical cardiac involvement, has only been reported once in the literature.