We evaluated cerebrospinal fluid biogenic amine metabolites in 32 patients with Rett syndrome. Significant reductions were noted in the metabolites of norepinephrine, dopamine, and serotonin, as compared with controls of similar age (p < 0.00l, p < 0.001, and p < 0.05, respectively). Tetrahydrobiopterin is an essential cofactor and may be rate‐limiting for the synthesis of catecholamines and serotonin. Accordingly, total cerebrospinal fluid biopterin was measured and found to be elevated in patients compared with controls (p < 0.002). These findings constitute the only biochemical changes detected in Rett syndrome so far.