In type 1 diabetes mellitus (type 1 DM), an eating disorder (ED) may occur, referred to as «diabulimia,» in which patients use insulin to control body weight, contributing to the decompensation of endocrinopathy. In the presented clinical case, a patient (35 years old) with a 10-year history of type 1 diabetes mellitus intentionally refuses regular insulin administration in order to reduce body weight. The psychopathological qualification of the phenomenon underlying the ED presented as diabulimia in this patient corresponds to grandiose notions as part of schizotypal disorder (F21). The peculiarity of the condition in this clinical case is that the grandiose notion of weight loss was observed before the manifestation of type 1 diabetes mellitus, and later, it was included in the structure of the internal presentation of the disease (endocrinopathy), led to manipulations with insulin therapy, which in turn resulted in late complications of diabetes.