Post-hemorrhagic hydrocephalus was studied in two homogeneous series including 410 patients, 320 of whom had ruptured intracranial aneurysms. Ventricular dilatation was more frequently observed in the patients with subarachnoid hemorrhage from ruptured aneurysm (44%) than in those in whom no vascular malformation was detected by the four vessel angiography (21%). Moreover, hydrocephalus was over twice as frequent in high grading (76-64%) than in low grade patients (31-35%). Ventricular dilatation was revealed in 58-52% of A.Co.A., in 39-30% of C.I.A. and in 20-21% of M.C.A. Altogether 51 patients underwent continuous ventricular drainage (EVD): 54% improved significantly, but fatal recurrent hemorrhage took place on drainage in half of them. On these grounds, it would appear that EVD may be indicated only in grade IV and, occasionally, in Grade III patients on condition that early radical surgery is carried out as soon as significant improvement is attained, without waiting for an entirely satisfactory grading. In this way, devastating hemorrhages on drainage could be prevented in the last patients of our series. Chronic disturbances of CSF dynamics calling for permanent shunting proved exceedingly rare in our patients.