A case of a 10-year-old boy with a congenital arachnoid cyst in the right middle cranial fossa is reported. The symptoms were precipitated by head injury of moderate intensity, occurred some days before the beginning of the clinical picture. The computerized tomography has demonstrated the existence of a simultaneous right fronto-parietal subdural hygroma that was responsible for the intracranial hypertension syndrome. The influence of the cyst in the hygroma formation has been suggested. The mechanisms of growth, the etiology, clinical aspects, the diagnostics and the surgical treatment of the arachnoid cysts are discussed.