About 4 cases of arterio-veinous malformations located within the occipito-cerebello-mesencephalic dihedral (which is this region of the brain including the cistern of Galen and the surrounding formations : the quadrigeminal bodies forwards, the splenium of the corpus callosum upwards, the upper face of the cerebellum and the pedonculus cerebellaris superior below) the authors related their own experience. These 4 cases are detailed from a clinical point of view. Them, a synthetic study is undertaken in which a comparison is done between the literature and the clinical features encountered : headache, sub-archnoid hemorrhage, impairment of consciousness, clinical features such as cranial nerve impairment, nystagmus motor impairment, cerebellar deficit, hydrocephaly. The value of angiography, ventriculography and CAT is discussed. All these patients have been operated upon as far as the authors think that a better chance can thus be offered to them. The choice of the technique is discussed. For one patient, the sub-temporal route was performed ; but it seems worth using the posterior inter-occipital route : this allows a better view on the lesion and an easier treatment of the malformation. These 4 patients suffered post operatively of a visual defect. The control angiography revealed no more arterio-venious malformation in two patients, a mild one in two others.