Introduction: Understanding the potential embolic source (PES) in ESUS patients may improve the diagnosis and treatment of such patients. Hypothesis: PES differ in young vs old patients with ESUS. Methods: Young patients (age 18-49) with ESUS who were admitted to our stroke center from 2006 to 2019 were identified retrospectively and matched with next consecutive older patients (age 50-99) with ESUS by admission date. PES were categorized as atrial cardiopathy, AFib diagnosed during follow up, left ventricular disease (LVD), cardiac valvular disease (CVD), PFO or Atrial septal aneurysm (ASA) and arterial disease. Patients who had cancer or a coagulopathy were excluded. Rate of types and number of PES and stroke recurrence were determined and compared between young and older patients. Results: In young patients (55.3% women, median age 39 years), the most common PES was PFO/ASA and rate of other PES was low. Most of young patients had a single PES (54.1%), only 9 of the young patients had multiple PES. In older patients (41.7% women, median age 74 years), the 3 most common PES were atrial cardiopathy, LVD, and arterial disease. More older patients had multiple PES than single PES. More young patients as compared to older patients didn’t have any PES identified. The rate of stroke recurrence was lower in young patients as compared to older patients, the difference approached but did not reach statistical significance. During a median follow up of 3 years, only 3 young patients had recurrent stroke, two of them had unclosed PFO. There were no recurrent strokes among young patients with no PES identified. Among 9 older patients who had recurrent stroke, 6 had atrial cardiopathy, and AFib was detected in 4 patients during follow up. Conclusions: PFO is the only common PES in young patients with ESUS, atrial cardiopathy is the most common PES in old patients with ESUS. Rate of other types of PES and stroke recurrence is low in young patients.