Report of two anatomical-clinical cases, one being an hydranencephalic and the other of congenital hydrocephaly. The neurological examination showed in both cases a similar picture which characteristics recalling those found in a normal newborn. In both cases a transillumination of the skull and a carotidangiography were performed in order to study the cerebral circulation. The conclusions are as follows: 1) Transillumination has not been safe method for differential diagnosis between hydrocephaly and hydranencephaly, since in case 2 (hydrocephaly) was obtained a clear transillumination whereas in case 1 (hydranencephaly) it was absolutely negative; it must be emphasized that the cranial fluid of the hydranencephalic case was cloudy thus explaining possibly the insuccess of transillumination. 2) No analogy was found between the arterial distribution demonstrated by angiography and the anatomical-pathological findings of the hydranencephalic case, i.e., the anterior and medial cerebral arteries were well visualized, but necropsy showed absence of the portions of the hemispheres that are supplied by those arteries (fronto-parieto-temporal portions); on the contrary the territory supplied by the posterior cerebral artery was quite well developed, but the this artery was reduced to a short, tortuous and slender tuft of vasa. 3) Owing to the impossibility to take EEG tracings in the hydranen-cephalic case it was not possible to evaluate the importance of EEG in the differential diagnosis between hydrocephaly and hydranencephaly.