A 13 1/2-month-old boy with severe microcephaly was found to have nearly total absence of the telencephalon. The patient had marmorated skin, hypoplastic penis and undescended testes. Spastic tetraparesis was present. Moro, grasp and sucking reflexes were easily elicited. He could not sit or stand, but was able to raise and support his head. He had occasional convulsions and a tendency to hypothermia and vomiting. The EEG showed symmetrical low-voltage theta-delta activity. His psychomotor development was severely retarded. Bone age was normal. Head circumference was 28cm at six months and did not increase after this age. At autopsy the small cranial vault and meninges were found to be intact. Brain weight was 105g. The supratentorial part of the brain was extremely small, consisting of an irregularly lobulated mass about 3cm in diameter and without any median fissure or ventricular cavity. The telencephalon was severely involved and partly replaced by gliomesenchymal scar tissue, while the diencephalic structures, including the eyes and the optic nerves and chiasm, were comparatively well-developed. The cerebellum and brain stem were essentially intact.