BACKGROUND: The development of immunotherapy has led to a paradigm shift in the treatment of malignant tumors. Immune checkpoint inhibitors (ICIs) function by blocking the receptors and ligands of T cells from binding one another, empowering them to target and attack cancer cells. ICIs along with other immunotherapy treatments, have seen a significant increase in usage in recent years. However, this increased utilization has been accompanied by a significant rise in immune-related adverse events (irAEs). While the majority of irAEs reported are linked to endocrine dysfunction, less common irAES affecting the gastrointestinal, skin, hepatic, pulmonary, and cardiac systems have also been reported. METHODS: In this report, we present a 72-year-old male patient with advanced non-small cell lung cancer in his left lobe, with 20% of their tumor cells positive for PD-L1. The patient's cells tested negative for actionable molecular markers, including as KRAS, EGFR, ALK, MET, and ROS1 alterations. RESULTS: In accordance with the guidelines of the National Comprehensive Cancer Network (NCCN), the patient was treated with a PD-1 inhibitor in combination with chemotherapy. Following treatment, the patient developed severe immunotherapy-related diabetes, which resolved after discontinuation of immunotherapy and the administration of insulin. CONCLUSIONS: This case highlights the potential complications of immunotherapy and underscores the critical role of experienced clinicians in recognizing the early warning signs of irAEs and effectively managing the resulting complications.