BACKGROUND: Recently, there has been an increase in the number of patients with multiple chronic diseases (MCD), particularly due to obesity and ageing. The role of type 2 diabetes mellitus (T2DM) in the development of MCD, however, is still unclear. AIMS: This study aimed to determine the incidence of T2DM in the structure of polymorbidity considering sex and age-related characteristics. MATERIALS AND METHODS: Patients with MCD (n = 2,254; 769 men/1,485 women; aged, 18-99 years) were examined. The incidence of type 2 diabetes among patients with MCD considering age and sex was determined. RESULTS: Type 2 DM was detected in 407 patients with MCD (18.1%; male:female, 1:2.53). The polymorbidity index in male patients with type 2 diabetes was 1.5-2.0 times higher than that in male patients without diabetes. The rate of polymorbidity index increase was similar in both groups; however, its high initial value in patients with diabetes at a young age determined the burden of the comorbidity at a later age. In type 2 diabetes, hypertension was the predominant comorbidity at 18-59 years of age (p