The article reports on 13 cases of agenesis of the perisylvian region in patients aged 2 to 53 years. The agenesis occurred in the temporal operculum in 7 cases, in the frontal operculum in 5 cases and the whole fronto-parieto-temporal region in one case. The agenesis was always associated with space requiring arachnoidal cysts. Symphyses and obstruction of the flow of the basal CSF spaces were also seen. In 85% of the patients were males. The patients tend to decompensate after light cerebrocranial trauma, and in many cases surgery reveals a subdural haematoma which fills the aplastic region, due to detachment of the exposed cerebral veins effecting basal drainage. In the remaining cases, surgery was performed because of general signs of cerebral compression, exarcebation of an existing disease associated with attacks, or because of the space occupying character of the concomitant arachnoidal cysts. In patients with agenesis of the perisylvian region without space occupying arachnoidal cysts, surgery is not performed for the time being, instead, regular neurological and CAT-Scan control is effected.