In young patients, the causes of ischemic stroke (IS) are substantially different from those in older patients. Understanding the causes of the disease in patients younger than 45 years is needed to elaborate examination algorithms used in the country’s vascular departments.Objective: to present the nosological characteristics of IS in patients younger than 45 years according to the hospital register of the regional vascular center.Patients and methods. The data of the hospital register of the neurological unit for patients with acute cerebral circulatory disorders, who were in the Perm Territorial Vascular Center, City Clinical Hospital Four, in 2014 to 2020, were analyzed. The analysis included 126 patients under the age of 45 years with IS verified by magnetic resonance imaging.Results and discussion. The pathogenetic subtypes of IS according to the TOAST criteria were large artery atherosclerosis [n=10 (7.9%)], cardioembolism [n=27 (21.4%)], small vessel disease [n=15 (11.9%)], IS of other determined etiology [n=23 (18.3%)], and IS of undetermined etiology [n=51 (40.5%)]. After excluding incomplete examination cases (n=29), the most common causes of IS were cardiac embolism (patent foramen ovale, atrial septal defect, prosthetic valves, acute infective endocarditis, and chronic cardiac aneurysm) (27.8%), large artery disease (atherosclerosis and unspecified arteriopathy) (21.6%), small artery disease (15.5%), as well as cervical and cerebral artery dissection (10.3%). Cryptogenic stroke was observed in 13% of patients. There was a preponderance of cryptogenic IS in the age subgroup of 21–25 years, cardiac embolism (mainly patent foramen ovale) in the subgroup of 26–35 years; the proportions of cardiac embolism, large and small artery diseases, and dissections were comparable in the subgroup of 36–40 years; large and small artery diseases prevailed in the subgroup of 41–44 years.Conclusion. The most common causes of IS in 18–44-year-old patients are cardiac embolism (mainly through the mechanism of paradoxical embolism) and large artery disease, including dissection. The etiology of IS depended on age: cardiac embolism prevailed in the younger age range, whereas large and small artery diseases were dominant in the older age range.