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To investigate the chemical pathology in the blood and lens, in cases of congenital or infantile cataract in children excreting predominantly non-reducing carbohydrates in urine.Urine samples from children with congenital or infantile cataract, and age and sex-matched controls, were analysed for (i) inherited errors of metabolism, (ii) paper chromatography of sugars, (iii) spectrophotometric assay of glycosaminoglycans (GAG), (iv) cetyl trimethyl ammonium bromide test, (v) electrophoresis using Alcian blue, (vi) ion exchange chromatography with IR 120 resin, and (vii) HPLC for xylose. Blood and lens material were also tested for GAG fragments and xylose. β Glucuronidase was assayed in lymphocytes and urine.Of 220 children of both sexes below 12 years of age, with congenital or infantile cataract treated in Sankara Nethralaya, Madras, India, during a period of 2 years, 145 excreted fragments of GAG (heparan and chondroitin sulphates) in their urine. There was no such excretion among the control group of 50 children. The same was found accumulated in the blood and lenses of affected children. In addition, xylose was present in small amounts in the urine and blood and xylitol was present in the lens. There was a significant elevation in the activity of β glucuronidase in lymphocytes and urine, when compared with normals. All the above findings suggest deranged proteoglycan metabolism. As the urine contained mostly GAG fragments and very little xylose, Benedict's reagent was not reduced. This ruled out galactosaemia.An increase of β glucuronidase activity might have caused extensive fragmentation of GAG with resultant accumulation in the blood and lens and excretion in urine. Small amounts of xylose may have come from xylose links between GAG and core protein of proteoglycans. Owing to their polyanionic nature, GAG fragments in the lens might abstract sodium, and with it water, thereby increasing the hydration of the lens. Excessive hydration and the osmotic effect of xylitol from xylose might cause cataract. While corneal clouding has been reported in inborn acid mucopolysaccharidosis, congenital or infantile cataract with deranged metabolism of proteoglycans (acid mucopolysaccharide-xylose-protein complex) is reported in children for the first time.