A retarded 8-month-old child was diagnosed clinically as a case of hydranencephaly with the help of transillumination of the head. Intracranial O2 injections revealed that the existing brain tissue consisted only of a small mass at the bottom of the cranial cavity. By simultaneous injection of O2 via the lumbar route, rudimentary lateral ventricles were demonstrated which did not communicate with the large cavity occupying almost the entire cranium. Cerebral angiography via the carotid arteries revealed an almost normal but "miniature" appearance of the internal carotid artery system and a normal external carotid artery system. These findings are completely different from those usually seen in cases of advanced hydrocephalus, and seem to indicate, in this case at least, that hydranencephaly is not the end result of severe internal hydrocephalus. Further studies of such cases with air and angiography are needed in order to differentiate between different types of conditions which are called hydranencephaly by clinical examination.