A 79-year-old patient with metastatic melanoma was treated with pembrolizumab. After the fifth cycle of the treatment, the patient presented two months later with a plasma glucose of 39.8 mmol/L (716.4 mg/dL) and Hb1Ac of 7.5%. The patient was diagnosed with PD-1 inhibitor autoimmune diabetes mellitus and managed with insulin. Here, PDCD1 is linked to metastatic melanoma.