The Rett syndrome is a progressive disorder in female patients that is characterized by autistic behavior, dementia, ataxia, loss of purposeful use of the hands, and seizures. The results of laboratory investigations are usually normal, with the exception of electroencephalography. In an attempt to understand the pathogenesis of this syndrome, we assayed biogenic amine metabolites in the cerebrospinal fluid of six patients, 2 to 15 years of age. 3-Methoxy-4-hydroxyphenylethylene glycol (MHPG, a metabolite of norepinephrine), homovanillic acid (a metabolite of dopamine), and 5-hydroxyindoleacetic acid (5-HIAA, a metabolite of serotonin) were quantitated by a method involving gas chromatography and mass spectrometry. Levels of MHPG and homovanillic acid were significantly reduced in all the patients as compared with those in controls of similar age (P = 0.01 and 0.008, respectively). The reduction in the level of 5-HIAA was not significantly different from that in the controls (P = 0.15). These findings suggest an underlying disorder of biogenic amines in the Rett syndrome and may provide new insight into its pathogenesis. (N Engl J Med 1985; 313: 921–4.). © 1985, Massachusetts Medical Society. All rights reserved.