Decompressive craniectomy is frequently used to treat increased intracranial pressure or an intracranial mass effect. Trephined Syndrome describes a neurological deterioration, which is attributed to a large craniectomy. The symptomatology is varied but includes headache, aggravation of a hemisyndrome or cognitive disorders, often has an orthostatic component and improves or disappears with cranioplasty. The incidence of Trephined Syndrome has been reported between 7% and 26%. However, it might be underestimated if the course of cognitive functions before and after cranioplasty were insufficiently documented.