Hurler syndrome: An inherited error of metabolism characterized by deficiency of the enzyme alpha-L-iduronidase, which normally breaks down molecules called mucopolysaccharides. Without the activity of this enzyme, mucopolysaccharides accumulate abnormally in the tissues of the body. There are two clinical subtypes of disease due to deficiency of alpha-L-iduronidase: Hurler syndrome and Scheie syndrome. Hurler syndrome patients have progressive mental degeneration, a broad forehead with heavy eyebrows, enlarged and deformed skull, small stature, corneal opacities, hepatosplenomegaly (enlargement of the liver and spleen), valvular heart defects, thick skin, joint contractures, and hernias. Hurler syndrome is inherited in an autosomal recessive manner. The gene that codes for alpha-L-iduronidase is on chromosome 4. Enzyme replacement therapy helps the body make alpha-L-iduronidase and can alleviate many of the symptoms, but enzyme replacement therapy has not been shown to affect the mental damage. Bone marrow transplantation may slow the progression of Hurler syndrome and may prevent mental retardation if done at an early age. Also known as mucopolysaccharidosis type I.