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Introduction Embolization of the middle meningeal artery (MMA) for non-operative acute subdural hematoma (aSDH) offers promise in preventing the development of membranes and chronic SDH (cSDH). Understanding the natural history of non-operative aSDH is vital for selecting the appropriate patient population characteristics for a clinical trial of MMA embolization in non-operative aSDH.Methods We conducted a retrospective chart review of adults (≥18 years) presenting with non-operative aSDH at our institution between January 2021 and June 2025. Our primary outcome was progression to cSDH on follow-up, defined as a persistent hematoma with a density less than 40 Hounsfield units on followup imaging 90 days after hospital discharge. Parafalcine and tentorial aSDH was excluded because they would not be amenable to MMA embolization. Univariate analysis was performed with t-tests, ANOVA, or non-parametric equivalents.Results Of the 3,049 preliminary patients screened for non-operative subdural hematoma, 88 met the inclusion/exclusion criteria. Within 90 days of discharge, 25 (28%) of included patients had a cSDH. Older patients were more likely to develop cSDH (p = 0.037). Developing cSDH was more prevalent in patients with a past medical history of cancer (p = 0.010) and kidney disease (p = 0.027). Imaging characteristics of aSDH correlated with cSDH transformation included increased hematoma volume (p < 0.001), anterior-posterior length (p < 0.001), thickness (p < 0.001), midline shift (p < 0.001), and the absence of a contusion (p < 0.030). No other recorded aSDH presentation characteristics (mechanism of injury, Glasgow Coma Scale, presentation symptoms, or laboratory findings) correlated with cSDH development. Patients with cSDH on followup were significantly more likely to neurologically decline (headache, gait instability, limb weakness, cognitive impairment, dementia, focal neurological deficit, and encephalopathy; p = 0.036) and a higher modified Rankin Scale (p = 0.044).Conclusion The risk of cSDH development after non-operative aSDH is significant and leads to deficits. Many aSDH patients did not fit the criteria of non-operative aSDH in a location amenable to MMA embolization or did not receive followup imaging, leading to a limited sample size. Larger datasets may find that older age and race predict cSDH recurrence. Larger Hematoma size, magnitude of midline shift, and presence of contusion strongly correlate with the development of cSDH and could be useful metrics for identifying which patients to include in future trials of MMA embolization for patients with non-operative aSDH.Disclosures H. Hutchinson: None. G. Hey: None. L. Bertola: None. J. Leclaire: None. B. Kent: None. M. Haghighi: None. C. DeYoung: None. S. Amini: None. M. Koch: None. A. Mittal: None.Abstract E-138 Table 1Selected differences between patients with or without cSDH development