Objective: Large vessel occlusion acute ischemic stroke (LVO-AIS) is considerably less common in young (18-50 years old) adults likely due to the elusive stroke mechanisms compared to older adults. We sought to assess the etiology of LVO-AIS in young adults presenting to Cleveland Clinic Stroke Enterprise using ASCOD classification system to identify underlying stroke etiology in those labeled as cryptogenic per TOAST classification. Methods: In this retrospective, single healthcare network study, we reviewed electronic charts of patients 18-50 years of age presenting with ischemic stroke between January 2017 and December 2021 to the Cleveland Clinic Stroke Enterprise. We identified patients who had a large vessel occlusion (LVO) on CTA or MRA at presentation. We excluded those without LVO or those who did not have intracranial vessel imaging available. We assessed demographics and stroke etiology using both the TOAST and ASCOD classification systems. We then compared those labeled as cryptogenic per TOAST classification with their respective ASCOD classification to elucidate further stroke mechanisms in these patients. Results: Out of 1175 patients who presented with acute ischemic stroke during the study period, 220 patients were found to have LVO-AIS and were included in the study. The mean age was 40.5 ±7.0. Median (Q1, Q3) initial NIHSS was 8 (3, 16). By TOAST, stroke of undetermined etiology (34.7%) was the most common followed by stroke of other determined etiology (28.6%). However, of those labeled as “undetermined etiology,” ASCOD classification revealed 7 (9.46%) as potentially causal and 7 (9.46%) as uncertainly causal etiologies. Conclusions: In our young adult stroke cohort, a third of patients had cryptogenic LVO-AIS. However, ASCOD classification revealed potentially causal and uncertainly causal etiologies that may have been overlooked in TOAST classification suggesting that ASCOD classification system may be more informative in establishing stroke etiology of LVO-AIS in young adults.