Arnold–Chiari I malformation is a congenital malformation of the hindbrain characterized by displacement of the cerebellar tonsils into the foramen magnum, pressure on the fourth ventricle, and decreased fluid flow to the basal cisterns. It is a mild form of the neuro-cranio-vertebral syndrome (Arnold–Chiari syndrome) because in many cases it is asymptomatic or has few clinical signs and symptoms of mild severity.The case study presented in the article concerns a 16.5-year-old patient with Arnold–Chiari I malformation following surgery. Based on the neuropsychological diagnosis, clinical symptoms that might result from the course of structural and functional changes in the hindbrain in Arnold–Chiari I syndrome were described and explained. The following were observed: selective difficulties in planning and concentration (maintaining the direction of action) and a weakening of visual perception (described by the patient as a subjective state of derealization).The results of the neuropsychological diagnosis point to certain practical implications for the further therapeutic treatment of patients with a mild form of Arnold–Chiari I malformation, especially as there is a risk of clinical symptoms changing over time due to the posibility of progressive neurodegenerative changes in the hindbrain.