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Introduction/Purpose The Woven EndoBridge (WEB) is the most widely used intrasaccular flow-disruptor, though conventional aneurysm occlusion scales required adaptation to its design, contributing to discrepancies between complete (53.8%) and adequate (85%) occlusion rates. The clinical significance of persistent filling remains unclear. This study compares observation versus retreatment of WEB-treated aneurysms with persistent filling, evaluating risks of rupture or growth and whether retreatment improves angiographic outcomes without increasing complications.Materials and Methods Retrospective data-collection of 16 high-volume cerebrovascular centers’ databases of patients treated with WEB was performed. Persistent filling was defined as either WOS C and D, BOSS 1, 2 and 3, or RR II and III. At the time of first follow-up after initial treatment, the outcomes were then categorized as either residuals, or as recurrences. Aneurysms in which persistent filling remained stable or upgraded but did not progress to complete occlusion were categorized as residuals. Aneurysms in which either complete occlusion or persistent filling displayed downgrading in these scales were categorized as recurrences. A visual schematic of these definitions is available on figure 1. Adults with documented aneurysm residual or recurrence after WEB-treatment were included. Clinical and angiographic outcomes of observation versus retreatment were collected for comparison pre and post propensity-score matching (PSM).Results Two-hundred-and-ten patients (observation, 124 vs retreatment, 86) were included. The retreatment cohort was younger (63.2 vs 58.6 years, p<0.01), more frequently current-smokers (25% vs 39.5%, p<0.01), with ruptured status prior to index treatment (12.9% vs 38.4%, p<0.01), and higher proportion of recurrences (27.9% vs 16.1%, p=0.04). Intact WEB position and appropriate size was more frequent in observation cohort (68.5% vs 43%, p<0.01), while WEB compression was more frequent in retreatment cohort (20.2% vs 38.4%, p<0.01). Observation cohort had higher rate of branches arising from aneurysm (21.8% vs 9.3%, p=0.02). Stent-assisted-coiling (53.5%) and flow-diversion (24.4%) were the most frequent modalities used for retreatment. Post-PSM, 56 pairs were generated. These were similar in regards to aforementioned variables, and remainder of demographics and aneurysms’ characteristics. Length of clinical and angiographic follow-up was not statistically different between cohorts. Retreatment achieved higher proportion (p<0.01) of RR I (19% vs 64.6%), and lower proportion of RR II (52.4% vs 22.9%) and RR III (28.5% vs 12.5%). Procedure-related complications occurred in 5.4% of retreatment cohort. Notably, none of the aneurysms in neither group suffered further rupture. The vast majority of patients in both the continued observation and retreatment remained functionally independent with mRS 0-2 (93.5% vs 91.1%, respectively, p=0.71).Conclusions Choice of retreatment versus observation appears to be guided by certain features such as age, comorbidities, prior rupture history, and anatomy such as branching vessels and device compression. When similar patients and aneurysms are compared, there was no difference in rates of further rupture, suggesting that despite incomplete occlusion, aneurysms are still secured after treatment with WEB likely due to flow-disruption with changes in dome hemodynamics. Furthermore, up to one fifth of aneurysms with persistent filling progress to complete occlusion with observation in the timeline of this study.Disclosures V. Jaikumar: None. A. Monteiro: None. R. Chase: None. J. Rios-Zermeno: None. A. Amllay: None. V. Sridhar: None. A. Karandish: None. O. Toledo: None. A. Aghaebrahim: None. E. Sauvageau: None. R. Calle: None. E. Levy: None.