We evaluated the course of neurocognitive functioning in newly diagnosed high-grade glioma patients and specifically the effect of tumor recurrence. Following baseline assessment (after surgery and before radiotherapy), neurocognitive functioning was evaluated at 8 and 16 months. Neurocognitive summary measures were calculated to detect possible deficits in the domains of (1) information processing, (2) psychomotor function, (3) attention, (4) verbal memory, (5) working memory, and (6) executive functioning. Repeated-measures analyses of covariance were used to evaluate changes over time. Thirty-six patients were tested at baseline only. Follow-up data were obtained for 32 patients: 14 had a follow-up at 8 months, and 18 had an additional follow-up at 16 months. Between baseline and eight months, patients deteriorated in information-processing capacity, psychomotor speed, and attentional functioning. Further deterioration was observed between 8 and 16 months. Of 32 patients, 15 suffered from tumor recurrence before the eight-month follow-up. Compared with recurrence-free patients, not only did patients with recurrence have lower information-processing capacity, psychomotor speed, and executive functioning, but they also exhibited a more pronounced deterioration between baseline and eight-month follow-up. This difference could be attributed to the use of antiepileptic drugs in the patient group with recurrence. This study showed a marked decline in neurocognitive functioning in HGG patients in the course of their disease. Patients with tumor progression performed worse on neurocognitive tests than did patients without progression, which could be attributed to the use of antiepileptic drugs. The possibility of deleterious effects is important to consider when prescribing antiepileptic drug treatment.