All Down syndrome (DS) children have different degrees of developmental disabilities, developmental delay, and developmental brain abnormalities associated with CNS maturation delay and cortical dysgenesis. We have examined 780 occipitofrontal circumferences (OFC), mean and +/- SD, of DS children from birth to age 5 years. Also, gross and microscopic neuropathological studies in the same age group were performed, with special attention to brain weight (BW), shape, myelin formation, cortical organization of 101 DS and 80 non-DS individuals; ultrastructural studies were also performed on selective cases (five DS and five non-DS). The OFC was plotted on Nellhause curves and showed microcranium after mid-infancy in most cases. Twenty percent of DS children had an OFC in the lower normal range. The brain shape in DS newborn infants was the same as in non-DS infants, but after 3-5 months of age in DS infants the antero-posterior diameter was found to be shorter than in non-DS infants. Narrowness of the superior temporal gyrus was noted in 34 of 101 (33%) of DS brains. Microscopic examination showed myelination delay in 22.5% DS and only in 6.8% non-DS children. Morphometric studies in DS cases from birth showed fewer neurons (20-50% less), lower neuronal densities, and neuronal distribution, especially of cortical layers II and IV. Ultrastructurally in DS, the synaptic density, synaptic length, and contact zones were found to be abnormal. The retardation of brain growth, maturation delay, and cortical dysgenesis present in DS children most likely are regulated by the extra chromosome 21, but the gene responsible for the abnormalities remains to be determined.