to assess the influence of diabetes mellitus (DM) on cognitive functions of metabolic syndrome (MS) patients. 305 patients aged 45-89 years with chronic cerebrovascular disease and MS were enrolled into the study. All patients were divided into two groups: without DM (n=148) and with DM (n=1 57), comparable for age and level of education. Metabolic syndrome and its individual components were assessed. All patients underwent neurological examinations and the application of neuropsychological tests to assess cognitive functions as well as laboratory tests and MRI. Their blood pressure was measured and the body mass index (BMI) was calculated. The carotid intima-media thickness (IMT), plaque and degree of stenosis were measured by a high resolution B-mode ultrasound examination. Patients with metabolic syndrome and DM had more pronounced cognitive impairment (CI) according to MMSE, compared with the patients without DM. According to the neuropsychological test results, metabolic syndrome and hyperglycaemia were more strongly associated with information processing speed and mental flexibility (both including perceptual speed) rather than with memory (immediate and delayed recall). Therefore, prompt and adequate treatment of the existing metabolic syndrome and its individual components, in particular diabetes mellitus, will reduce the risk of both moderate CI and vascular dementia and improve the patients'quality of life.