Malignant gliomas are brain tumors with dismal prognosis that can affect patients’ neurocognitive status. We performed longitudinal neuropsychological assessments to describe variations due to illness progression and treatment administration. Patients underwent a battery of standardized neuropsychological tests tapping into different cognitive domains (memory, attention, abstract reasoning, executive functions, learning), pre-radiotherapy and at 6, 12 and 24 month follow-up. We enrolled 130 patients, and almost 70% of them had at least one cognitive deficit. The most affected domains were executive function and long- and short-term memory. At follow-up assessments, cognitive status worsened in one-third of patients, whereas it remained unchanged or improved in two-thirds of patients. This is one of few longitudinal studies investigating cognitive function in a large sample of patients to monitor changes along the illness course.