Background Isolation was implemented for elderly patients with coronavirus disease 2019 (COVID-19), which interrupted care including oral intake support. ObjectiveTo assess the changes in feeding and swallowing function measured by the functional oral intake scale (FOIS) and their association with post-discharge mortality in elderly COVID-19 patients. Design Single-center retrospective cohort study. Participants We included patients aged >= 65 years and admitted for COVID-19 between December 1, 2021 and March 31, 2023. Those with FOIS before onset (pre-FOIS) 1-3 were excluded. Main Measures We measured FOIS (1-7, 1: no oral intake, 7: normal) before onset and at discharge, and assessed mortality for 180 days after onset. The primary outcome was decrease in FOIS during hospitalization; in-hospital mortality was assumed as 4-level decreases in FOIS. The secondary outcome was 180-day mortality. Association of each pre-FOIS (6, 5, 4) relative to pre-FOIS 7 with the primary outcome was estimated as adjusted common odds ratios (ORs) and 95% confidence intervals (CIs). Association of decreases in FOIS with 180-day mortality was assessed with Kaplan-Meier curve. Key Results We included 337 patients whose median age was 81 years, and 56% of them were men. The distribution of pre-FOIS was 7 (190 patients), 6 (89), 5 (40), and 4 (18). The severity of COVID-19 was generally similar among pre-FOIS levels. The pre-FOIS was significantly associated with decreases in FOIS: adjusted common OR 2.23 [95%CI 1.27-3.92] for pre-FOIS 6, 2.96 [1.46-6.05] for pre-FOIS 5, 2.89 [1.14-7.40] for pre-FOIS 4. The degree of decrease in FOIS was significantly associated with 180-day mortality: no decrease, 4.7%; 1-level decrease, 1.2%; 2-level decrease, 27.9%; 3-level decrease, 46.0%. Conclusions Lower pre-FOIS was associated with further decreases in FOIS in elderly patients who were isolated due to COVID-19, and the degree of decrease was further associated with post-discharge mortality.