This paper presents a realistic evaluation of the prevalence of type 2 diabetes mellitus (T2DM) among Polish schizophrenic patients who have sought treatment through the Polish National Health Fund in the years 2010-2017. Data from the National Health Fund database was used and T2DM and schizophrenia groups were defined according to International Classification of Diseases (ICD-10) codes. Demographic data were collected from the web page of Statistics Poland (GUS). The annual prevalence of T2DM and schizophrenia was estimated, and the age groups were categorised into eight sets. The incidence of schizophrenia in T2DM patients in the years 2010-2017 was measured, including relative risk and 95% confidence interval (95% CI). The incidence of T2DM has been assessed in various subtypes of schizophrenia. In the eight years of follow-up study, 1,481,642 patients with schizophrenia were included, of which 185,205 were also diagnosed with T2DM. This accounted for 12.50% of all patients with schizophrenia. The trend of comorbid schizophrenia (F20) and T2DM (E11) in the general population of patients with schizophrenia, who sought treatment through the National Health Fund, was relatively stable in the years 2010-2017. The relative risk of T2DM in those with schizophrenia was 8.33 (95% CI 8.23-8.43) in 2017. Taking actions to enable the detection of diabetes in patients with concomitant schizophrenia is well-grounded, although these actions should be gender-dependent. There is also a need to take adequate actions to improve the efficiency of diabetological care among patients with schizophrenia.