Diabetes mellitus with severe diabetic ketoacidosis developed 2 weeks after the seventh infusion of anti-PD-1 monoclonal antibody with a characteristic clinical picture (nausea, vomiting, polyuria, polydipsia) and biochemical parameters (glucose 38.6 mmol/l, ketones 4.9 mmol/l (<0.6), C-peptide <0.01 μg/l (1.1–4.4), HbA1c 8.3%), but negative serological — Ab to GAD and AT to ICA were given). This evidence concerns the gene GAD1 and diabetes mellitus.