ObjectivesThe coronavirus disease 2019 pandemic significantly affected Japanese society and the health of its population. Despite this, few studies have evaluated the influence of the pandemic on patients with neurological diseases or dementia, which we assessed through the Tochigi Dementia Cohort Study. MethodsParticipants were divided into two groups. The pre-pandemic group included patients who were enrolled from December 1, 2016 to November 30, 2018, and were followed up until November 30, 2019 (i.e., before the pandemic). The post-pandemic group included patients who were enrolled from December 1, 2019 to November 30, 2021, and were followed up until November 30, 2022 (i.e., during the pandemic). We recorded their age, sex, mortality, and treatment withdrawal during the follow-up period. Furthermore, we examined their cognitive function at the baseline, and after 6 and 12 months. ResultsA total of 384 patients were enrolled in this study, including 199 patients in the pre-pandemic group and 185 in the post-pandemic group. The mortality of dementia patients was significantly higher in the post-pandemic group than in the pre-pandemic group” (5.3% vs. 18.5%, p < 0.05*). The cognitive function scores at 12 months were also significantly lower in the dementia patients of the post-pandemic group than in those of the pre-pandemic group (p < 0.05*). ConclusionsThis longitudinal cohort study conducted in a local Japanese area revealed that mortality rate and cognitive function worsened in dementia patients during the pandemic.