Background: PRO enable direct measurement of the experiences of patients with cancer. Anti-programmed death 1 (PD-1) therapies have shown favorable PRO in lung cancer but a data gap remains in EC. Dostarlimab is an investigational anti-PD-1 monoclonal antibody which has shown promising activity in GARNET in advanced mismatch repair deficient (dMMR) EC pts (with an ORR of 42.9% and a disease control rate of 58.6%), and a low incidence of symptomatic grade ≥3 treatment-related adverse events (anemia [2·9%], colitis [1·9%], and diarrhea [1·9%]). Here, we report on PRO measures collected from pts with dMMR/microsattelite instability high (MSI-H) EC in the single-arm GARNET trial. Methods: Pts with dMMR/MSI-H EC confirmed by local tests, with recurrent or advanced disease that progressed on a platinum regimen were enrolled. Pts received 500 mg Q3W of dostarlimab for 4 cycles, then 1000 mg Q6W until disease progression or discontinuation. PRO assessment was an exploratory endpoint and was measured by the EORTC Quality of Life Questionnaire (QLQ-C30), a validated instrument used to evaluate quality of life, functioning, disease symptoms, and treatment-related side effects. PRO were collected at each dose administration, end of treatment, and follow-up. A mixed-model for repeated measures was used to assess change from baseline, accounting for time and baseline ECOG scores. The threshold to determine clinically meaningful group-level change was ±10 points. Results: PRO data were available for 43 pts. Compliance rates were high at 98%. Relative to baseline, pts reported meaningful improvements in pain, insomnia, and social and emotional functioning over the trial duration. Appetite, nausea, vomiting, constipation, diarrhea, and physical and role functioning were stable over the trial duration. Quality of life and global health status were also maintained. Conclusions: PRO from 43 pts enrolled in the GARNET trial show that disease- and treatment-related symptoms and quality of life are improved or maintained while receiving treatment. These data, along with with the efficacy and safety profile of dostarlimab, strongly support the use of dostarlimab in dMMR/MSI-H advanced EC. Clinical trial information: NCT02715284