Children with an X-linked neurological disease often classed as cerebral palsy show an absence of an enzyme of purine metabolism, hypoxanthine-guanine phosphoribosyltransferase (PRTase), in the brain, liver, fibroblasts, and erythrocytes. The absence in these patients of PRTase activity in the basal ganglia where the enzyme is normally of highest activity can be correlated with the fact that the major clinical symptoms are attributable to basal ganglia dysfunction. The concentration of oxypurines (hypoxanthine and xanthine) in the cerebrospinal fluid (CSF) was four times normal and was greater than in plasma, which suggests that the brain also has an increased purine synthesis. The possible role of high oxypurine concentration in CSF in development of the neurological disease has been discussed. Treatment with allopurinol produced a further increase in the concentration of oxypurines in CSF.