We analyzed neuronal phenotypes and dendritic growth in the newborn with semilobar holoprosencephaly and 18p deletion. We found that the holoprosencephalic neocortex retained its basic six-layered lamination but displayed a number of intralaminar and modular architectonic alterations and contained a mixture of normal and aberrant neuronal phenotypes. The most conspicious finding was the presence of the pronounced increase in soma size and total basal dendritic length of holoprosencephalic layer III pyramidal neurons in comparison to age-matched control brains. The dramatic (5-fold) dendritic overgrowth observed in associative cortico-cortical pyramidal neurons is probably related to the pronounced diminution of the cortical afferent input.