Immunotherapy targeting T-cell regulatory molecules is highly effective in multiple cancers refractory to standard chemotherapies. However, blocking inhibitory molecules on activated T cells not only increases tumor cell destruction but also can breach tolerance, enabling pathological T cells to react with self-antigens. Indeed, autoimmune endocrinopathies, including hypophysitis, hypopituitarism, and thyroiditis, have been reported in trials involving anti-CTLA-4 and anti-PD-1 monoclonal antibodies (1–3). But autoimmune diabetes has not been definitively linked to these agents. We describe the development of new-onset insulin-dependent diabetes in five patients after receiving anti-PD-1 antibodies, either as single agent or in combination with other cancer drugs. Clinical history and key laboratory findings are summarized in Table 1. Notably, while the patients presented with diverse cancer types, and some had been treated with other immunological agents, their histories were common for anti-PD-1 antibody exposure prior to developing autoimmune diabetes. Time from drug administration to diabetes onset spanned 1 week to 5 months, when patients presented with severe hyperglycemia or diabetic ketoacidosis (DKA) with elevated HbA1c. Diabetes was a new diagnosis for all but one patient who had preexisting type 2 diabetes controlled with metformin. Most patients exhibited inappropriately low or undetectable C-peptide (Table 1). All were initiated on insulin therapy upon presentation and remained insulin-dependent for glucose control. Table 1 Clinical history and key laboratory findings Patient Age/sex Primary diagnosis Pertinent history Anti-PD-1 drug Other chemotoxins Diabetes presentation Random C-peptide* and glucose Time after anti-PD-1 Antibody positivity/titers^ HLA Diabetes antigen-specific T cells† 1 55/F Melanoma Autoimmune thyroid disease Nivolumab Ipilimumab, prednisone DKA, glucose 532 mg/dL, HbA1c 6.9% (52 mmol/mol) 55 years). Not only do our cases demonstrate temporal correlation between anti-PD-1 treatment and diabetes onset, they also provide the first mechanistic support for cancer immunotherapies targeting T-cell regulatory pathways to precipitate autoimmune diabetes. Other factors that may influence predisposition for hyperglycemia and autoimmunity in our patients included combined use with other immune modulators (patient 1), pancreatic metastases (patient 3), and preexisting type 2 diabetes (patient 4). Nonetheless, the fact that they all developed acute severe hyperglycemia with ketoacidosis or low/undetectable C-peptide levels is strong evidence for a new and insulin-deficient type of diabetes. Diabetes had previously been reported as an adverse event to anti-PD-L1 (2) and one case was reported in 206 subjects treated with nivolumab (3), but there lacked evidence for an autoimmune mechanism. Our report demonstrates humoral and cellular autoimmunity in multiple patients with anti-PD-1–induced diabetes. While it is difficult to estimate the true incidence of this phenomenon, the five patients in our series represent less than 3% of total subjects who have participated in PD-1/PD-L1 trials at our institution. These cases illustrate the importance of recognizing this potential precipitant of autoimmune diabetes in older individuals receiving immunotherapy.