The results of this study demonstrated the feasibility of the HVLT-R Japanese version for evaluating the patients’ NCF after brain irradiation. We found that the patients with either older age or lower KPS had impaired NCF as assessed with the HVLT-R at the baseline. The four-month NCF deterioration was associated with both the KPS and poor outcome. The WBRT dose fractionation did not have an impact on the incidence of the decline in the NCF at four or eight months. The early decline in the NCF is characteristic of the worse-prognosis group, and thus the NCF of favorable-prognosis patients might better be evaluated later (i.e., at six months or later) after irradiation.