Immune checkpoint inhibitors have improved survival rate in patients with advanced melanoma, but also have the potential to induce several adverse events. We report on a 63-year-old woman who had advanced melanoma and was admitted with diabetic ketoacidosis, which had occurred upon treatment with ipilimumab. On admission, the C-peptide level was low, and the HbA1c concentration was 50 mmol/l indicating a rapid onset of the disease. The patient had also been diagnosed with thyroiditis. Diabetes mellitus is a rare and serious side effect of treatment with ipilimumab, and we recommend being aware of this due to the rapid course.