Neuropsychologic studies were performed in 49 adult patients with primary intracranial tumors following surgery and radiation therapy. In the absence of computed tomography (CT) scan evidence of tumor recurrence or neurologic disorders, patients' intellectual functioning was tested once (49 patients), twice (18 patients), or three times (5 patients) by the Wechsler Intelligence Scale (WAIS) between 1.5 and 110 months after the beginning of irradiation. Full scale IQ (FSIQ) and deterioration coefficient (DC) were evaluated. Time elapsed from treatment to initial evaluation (vertical study) determined three statistically different patient groups. During the first 4 months, DC and FSIQ did not differ from a normal population. From 5 to 30 months, there appeared a significant probability of intellectual deterioration (p = 0.01), whereas median FSIQ remained at a normal level. Beyond 30 months, deterioration disappeared in terms of DC, but median FSIQ fell (p = 0.01). Second and third evaluations (horizontal study) confirmed the initial evaluation. Nevertheless, later recovery from FSIQ impairment was seen in 4 of 5 patients who were less than 30 years old and in 2 patients between 30 and 50 years of age who returned to work early after treatment. We believe that early neuropsychologic testing to detect early psychologic impairment is needed in all adult patients treated for brain tumors, especially in younger patients. Psychologic assistance and an early return to work may be beneficial.