The biochemical findings in 29 patients with Sanfilippo disease are reported and a scheme for laboratory diagnosis is outlined. A grossly elevated urinary excretion of heparan sulphate was a consistent and diagnostic finding, even at birth. The excretion of heparan sulphate and chondroitin sulphate was quantitatively similar in types A and B of the condition. Modifications of previously described methods for the determination of heparin sulphamidase in leucocytes or skin fibroblasts and N-acetyl-alpha-D-glucosaminidase in plasma or fibroblasts facilitated the measurement of specific activities. Sanfilippo A disease appeared to be the commonest mucopolysaccharidosis occurring in England and Sanfilippo B disease, one of the rarest forms.