Abnormal architecture of the telencephalic cortex was studied in six autopsy specimens of alobar holoprosencephaly by histopathological, electron microscopic and immunohistochemical methods. Several common abnormalities are described and their significance discussed. In all of the specimens, the cortices showed excessive thickness and abnormal lamination with a middle cellular layer and an external sparse layer, indicating disturbance of neuronal migration. Neurons of the external sparse layer were segmented into irregularly arranged groups, suggesting disturbance of tissue organization. In three specimens, the internal pyramidal layer contained numerous abnormal glomerular structures which consisted of fine dendrites and axons.