The purpose of this study is to report a case of a very large arachnoid cyst in a patient having spent a very active life and who had never presented any neurological or psychiatric symptomatology beside a dementia in the last years of her life. The gross examination of the brain revealed the presence of a voluminous frontal bilateral cyst, located in the midline and displacing laterally the frontal lobes, so they displayed a foliated aspect. Microscopically, the examination of the cyst walls confirmed its arachnoid origin and the cerebral cortex contained lesions typical for senile dementia. This case exemplifies the histological nature and the pathogeny of arachnoid cysts, in particular of congenital origin. This also shows that the very early occurrence of such a malformation does not prevent the development of functional neuronal pathways, owing to the important plasticity of the central nervous system.