We report two types of venous patterns associated with great cerebral vein (vein of Galen) absence or unavailability. Developmental venous anomalies or vein of Galen arteriovenous malformations (VGAM) serve as an illustrative material. A diencephalic pattern that collects the thalamo-striate veins into the tentorial sinus is recognized in most VGAM. A telencephalic arrangement connecting the striate veins with the rostral afferents to the basal vein is less frequent. Both patterns reproduce embryonic stages preceding the development of the great cerebral vein, thus confirming Raybaud's hypothesis that in VGAM the pouch is not the vein of Galen but the medial vein of the prosencephalon. The prognostic value of each pattern can then be appreciated and the therapeutic strategies rationalized; some unexplained complications of the venous approach for non selected VGAM can thus be avoided.