BACKGROUND AND OBJECTIVESIn patients with acute ischemic stroke (AIS), the impact of hemorrhagic transformation (HT) after endovascular treatment (EVT) on poorer stroke outcome is well established when associated with clinical deterioration. However, the influence of asymptomatic HT remains unclear. We aimed to examine the impact of asymptomatic HT after EVT on functional outcome and mortality.METHODSDrawing on Catalan (Spain) population-based prospective stroke registry data, we included patients from 10 comprehensive stroke centers with anterior circulation AIS (2017-2023) who underwent EVT, excluding patients without data on the presence of HT or functional outcome at 3 months of follow-up. HT was categorized as parenchymal hemorrhage (PH), hemorrhagic infarct (HI) types 1 and 2, and remote PH (rPH). Asymptomatic HT was defined as any HT not causing death or the NIH Stroke Scale (NIHSS) score to increase by ≥ 4 points. Functional outcome was centrally assessed using the modified Rankin Scale (mRS). The primary end point was a shift in the 3-month mRS score. After excluding symptomatic intracerebral hemorrhage (sICH), multivariable ordinal regression analyses (adjusted by age, mRS, baseline NIHSS score, baseline Alberta Stroke Program Early CT Score, and modified Thrombolysis In Cerebral Infarction score ≥2b) were performed to test for asymptomatic HT association with the primary end point.RESULTSWe included 3,067 patients (72.0 ± 13.6 years, 50.7% women), 179 (5.8%) with sICH and 612 (20.0%) with asymptomatic HT. HT category frequencies were 8.9% HI1, 7.2% HI2, 4.4% PH1, 3.8% PH2, and 1.5% rPH. The percentage of asymptomatic patients showed a hierarchical distribution, ranging from 93.4% in HI1 to 25.0% in PH2. In the multivariable analysis, asymptomatic HT was associated with poorer outcomes (common odds ratio [cOR] 2.24, 95% CI 1.89-2.66) and higher mortality (adjusted odds ratio 1.50, 95% CI 1.17-1.91). In the sensitivity analyses, the association with functional outcome remained significant for each HT category, with asymptomatic PH2 showing the highest odds of poorer outcomes (cOR 3.15, 95% CI 1.46-6.83).DISCUSSIONIn patients with AIS undergoing EVT, asymptomatic HT was associated with poorer functional outcomes and higher mortality, suggesting that any HT, regardless of its clinical impact or radiologic category, should be considered as an additional EVT safety measure.