Immune checkpoint inhibitors (ICI), particularly antibodies targeting programmed death receptor-1 (PD-1) and its ligand, programmed death-ligand 1, have been rarely associated with the development of immune-mediated diabetes mellitus. To the best of our knowledge, we report the first published case of immune-mediated diabetes mellitus associated with dostarlimab—a monoclonal antibody targeting PD-1—in a 78-year-old woman diagnosed with endometrial adenocarcinoma and persistent peritoneal disease. Following six cycles of dostarlimab, she presented with diabetic ketoacidosis. Despite negative autoimmune antibodies, her relatively low HbA1c and C-peptide levels together with her clinical presentation were highly suggestive of ICI-induced diabetes mellitus. Following stabilization with intravenous fluids and insulin infusion, she was transitioned to a basal-bolus insulin regimen. Dostarlimab was discontinued, and restaging showed resolution of peritoneal disease. This case emphasizes the importance of early detection and management of ICI-induced diabetes mellitus.

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