Type 1 diabetes mellitus is a condition caused by autoimmune damage to insulin–producing beta cells of the pancreatic islets, leading to endogenous insulin deficiency. Despite the sufficient knowledge of the disease and the availability of clinical recommendations for substitution therapy, the number of patients with this pathology is growing worldwide. At the same time, their cohort is very heterogeneous, including amid different etiology, concomitant genetic background, variations in the manifestation of the disease and severity. In this regard, traditional ideas about type 1 diabetes mellitus are being questioned, which requires special attention when managing patients with a clinical picture of the disease that differs from the traditional one. The article presents a clinical case of type 1 diabetes mellitus in a young patient, which demonstrates the importance of a personalized approach to the diagnosis and treatment of diabetic patients with a "non-classical" history.