The Sanifilippo syndrome is an inherited dementia caused by defective degradation of heparan sulfate. In the course of its catabolism the heparan sulfate polymer must be desulfated. Heparan sulfate sulfatase activity was demonstrated in homogenates of normal tissues and cultured skin fibroblasts, and in normal urine. This activity was found to be grossly depressed or absent in necropsy specimens of liver and spleen from two Sanfilippo patients. The heparan sulfate sulfatase activity was not demonstrable in urine from eleven, or cultured fibroblasts from four Sanfilippo patients. Activities of alpha-N-acetyl-glucosaminidase, the site of the metabolic defect in the Sanfilippo B variant were either normal or slightly elevated in the Sanfilippo tissues and cultured fibroblasts whereas the mean level in the urine of our Sanfilippo patients was about one-third of that encountered in control urines.