Introduction. Insulin allergy is a rare complication of insulin therapy, especially in type 1 diabetes mellitus (T1DM). Key manifestations are hypersensitivity-related symptoms and poor metabolic control. T1DM, as well as insulin allergy, may develop in the context of autoimmune polyendocrine syndrome (APS), further complicating management. Case Report. A 17-year-old male patient, diagnosed with T1DM, was treated with various insulin therapy schemes over several months, which resulted in recurrent anaphylactoid reactions and poor glycemic control, after which he was referred to our Endocrinology and Immunology Department. A prick test was carried out for all commercially available insulin presentations and another insulin scheme was designed but proved unsuccessful. A desensitization protocol was started with Glargine alongside administration of Prednisone, which successfully induced tolerance. Observation of skin lesions typical of vitiligo prompted laboratory workup for other autoimmune disorders, which returned positive for autoimmune gastritis/pernicious anemia. These findings are compatible with APS type 4. Discussion. To our knowledge, this is the first documented case of insulin allergy in type 4 APS, as well as this particular combination in APS. Etiopathogenic components shared by insulin allergy and APS beg for further research in immunogenetics to further comprehend pathophysiologic aspects of these diseases.