We have found normal concentrations of neopterin, monapterin, isoxanthopterin, biopterin and pterin in the urine of 10 patients with Rett syndrome, and normal values for total biopterin and neopterin in the blood of 4 subjects. Thus, there is no biochemical evidence of a generalized tetrahydrobiopterin deficiency in this syndrome. Since we had no opportunity so far to study cerebrospinal fluid, a defect in the metabolism of pterins in the central nervous system is not yet fully excluded.