Rett syndrome, the commonest condition associated with severe mental retardation in girls, is diagnosed only by its clinical phenotype, because, to date, there is no consistent characteristic alteration in genetic, biochemical, neurotransmitter or morphologic marker. The clinical features at various ages suggest involvement of most parts of the nervous system, however, the brain in Rett syndrome is reduced in weight, without other obvious morphologic alterations. Because of the relative microcephaly, hypotheses regarding failure of development have been suggested. Supporting such hypotheses are the quantitative studies by Jellinger, Seitelberger and Kitt defining a decrease in the amount of melanin in the substantia nigra and by Bauman defining a global decrease in the size of the neurons. In this study the cerebral cortex has been examined using the rapid Golgi technique with the purpose of investigating dendrites of pyramidal neurons in six cortical regions of Rett girls from ages 2.9–35 years. Camera lucida drawings of apical and basal dendrites of two cortical layers and CAI were prepared. These were submitted to the Sholl analysis. The Sholl analyses were tested for significance using the repeated measures analysis of covariance, with age as a covariate. The studies demonstrate that from our samples there is no evidence that the pyramidal neurons in Rett syndrome degenerate progressively with increasing age, but that the basal dendrites of layers three and five pyramidal neurons in the motor and frontal cortex, the apical dendrites of layer five of the motor cortex, and the basal dendrites of layer four of the subiculum are significantly shorter than in non-Rett brains. The dendritic trees in the visual cortex are not significantly decreased. This selective, non-progressive involvement of projection neurons of motor, association and limbic cortex may have bearing on the neurologic deficits in Rett syndrome, and these areas of the brain should be investigated to search for abnormalities of trophic factors in Rett syndrome.