We quantitatively analyzed the dendritic and dendritic spine development on basal and oblique dendrites of large layer IIIc pyramidal neurons of the prospective prefrontal area 9 in the brains of three infants with Down syndrome (DS) and five age-matched-controls over the period from 32 weeks postconception to the 7th postnatal month. By using Neurolucida 3.1 software on rapid Golgi impregnated slices, 9-10 neurons were three-dimensionally reconstructed. There were no significant differences in the pattern of the dendritic and spine development between the basal and apical oblique dendrites. The DS subjects did not depart significantly from the developmental curve of the control subjects. Our data showed that large and significant segment outgrowth, in parallel with dendritic elongation occurred during a limited period of time, between 36 weeks postconception and the first postnatal month. Dendritic spines appeared at the time of birth and their density continued to increase up to the age of 7 months. During the first postnatal month long thin spines and filopodia-like protrusions predominated, but the spines later changed their morphology to a more mature form. No differences in the spine morphology were qualitatively observed between the DS infants and the age matched controls. This data suggests that intensive formation of cortical circuitry occurs on large layer IIIc pyramidal neurons during perinatal period and is not disturbed in DS infants. Consequently, this could be a biological potential to mitigate psychomotor impairment in DS patient.