Abstract Disclosure: H.F. Belal: None. N. Shaykh: None. A. Rajesh: None. J.M. Chehade: None. Background: Pembrolizumab is a programmed cell death protein 1 inhibitors (PD-1i) approved for the treatment of many solid cancers. Although thyroid abnormalities are among the most common endocrine adverse effects associated with PD-1i, insulin-dependent diabetes mellitus can also occur in up to 1% of patients treated with PD1-i.[1] Clinical Case: A 70-year-old female with past medical history of non-toxic multi-nodular goiter and surgical hypothyroidism was diagnosed with gastric cancer. She had no pancreatic abnormality on PET scan. Her management included four cycles of neo-adjuvant chemotherapy: fluorouracil, cisplatin, trastuzumab and pembrolizumab. She had no personal history of gestational diabetes. Prior to initiating neo-adjuvant chemotherapy her A1c was 5.9%. She was not prescribed any anti-diabetic medications. Her grandmother had type 2 diabetes mellitus. On exam her BMI was 23.73 kg/m². She had no acanthosis nigrans, skin tags or abdominal striae. Eighteen days after completing her second cycle of neo-adjuvant chemotherapy, she presented to the emergency department with nausea, vomiting, polyuria and confusion and was found to be in DKA. Her A1c during admission was 7.1%. Imaging revealed no pancreatitis. Labs showed suppressed C-peptide <0.1 ng/mL (1.1-4.4) in the setting of blood glucose 283 mg/dL (71-99). Her GAD-65 antibody was elevated at 902 U/mL (0.0-5.0). She was diagnosed with PD-1i induced autoimmune diabetes mellitus (PD-1i DM) and subsequently treated with basal and prandial insulin. Conclusion: The mechanism of PD-1i DM is unclear. A proposed mechanism is aberrant T cell activation towards pancreatic islet cells. One study found that 59% of patients with PD-1i DM presented with DKA.[1] In this study, the median time of PD-1i DM onset was 20 weeks after the first treatment cycle. Additionally, 40% of these patient had at least one positive pancreatic autoantibody. A recent meta-analysis found the relative risk of PD-1i DM to be 1.56 (95% CI = 0.95-2.57, P = 0.08).2 This relative risk increased to 2.06 (95% CI = 0.48-8.86, P = 0.33) when PD-1i were combined with cytotoxic T lymphocyte antigen-4 inhibitors.