Autopsy material from three patients with Rett syndrome and six control cases (frontal, temporal and motor cortical areas) were studied. Pyramidal and non-pyramidal neurones, 100-200 in each specimen, were visualized by Lucifer Yellow microinjection. 3-D reconstructions and rotations were computed using the VoxelView/GT program. Various types of abnormalities in 3-D dendritic architecture of pyramidal neurones in different cortices were observed. Some pyramidal cells in layers II-III had a subnormal number of dendrites. The normally occurring specializations in pyramidal architecture in different cortical areas were absent. Decreased numbers and regional loss ('naked' areas) of dendritic spines were found. Widely distributed cortical afferents were shown by the carbocyanine dye (DiI) tracing method. PAD showed no microdysgenesis, in contrast to the observations in epilepsy.