Rett syndrome is now considered to be a neurodevelopmental disease. Its cause is unknown, but it has been suggested that neuronal growth factors and neurotransmitters play important roles. We measured levels of brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor in cerebrospinal fluid, and nerve growth factor and brain-derived neurotrophic factor in serum in child and adolescent patients with Rett syndrome. Levels of brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor in cerebrospinal fluid were below the limit of sensitivity of the methods used. Serum levels of nerve growth factor and brain-derived neurotrophic factor did not differ from control values. In Rett syndrome, the normal serum levels of nerve growth factor together and previously reported low levels of the factor in cerebrospinal fluid indicate that the latter may reflect low levels of nerve growth factor in the central nervous system. (J Child Neurol 1998; 13:429-433).