The callosal defect (CD) is of little clinical importance in itself. Therefore, the psychomotor symptoms frequently accompanying it are probably caused by associated brain anomalies. The more common of these are encephaloclastic lesions and malformations based on retarded migration. The frequency with which they occur makes them an important part of the syndrome, especially in regard to the interpretation of the symptoms and diagnostic signs, e.g. the anatomical and pneumoencephalographic appearances of the brain in CD. The picture of “clean” (i.e. uncomplicated) CD may thus be considerably modified or, with respect to isolated features, imitated by the effects of the associated brain lesions. Thus microcrania and microencephaly can be explained on the basis of the associated lesions, patients with “clean” CD having a skull and brain of normal size. A more limited and selective hypoplasia due to focal destruction and malformations is more common. Both cortical abnormalities and lesions of the white matter accentuate the callosal defect by way of tract hypoplasia, reduction in size of the longitudinal bundle and dorsal extension of the lateral ventricles-all features of importance in the diagnosis of CD. These associated lesions may be responsible for additional features not seen in clean CD such as brainstem hypoplasia due to corticospinal tract deficiency, ventricular widening and deformity. Consequently the associated lesions are of great importance from the clinical and diagnostic viewpoints.