Purpose <p>Endovascular treatment (EVT) for large ischemic stroke is associated with a risk of hemorrhagic transformation (HT). The severity of ischemic stress can be assessed using the apparent diffusion coefficient (ADC), with a high ADC indicating viability. This study aimed to examine whether a low ADC is associated with HT following EVT, based on a low Alberta Stroke Program Early Computed Tomography Score (ASPECTS) and diffusion-weighted imaging (DWI).</p> Methods <p>This retrospective cohort study included consecutive patients with acute large ischemic stroke (ASPECTS-DWI ≤ 5) who underwent EVT with successful recanalization between April 2014 and March 2023. The most frequent ADC (peak ADC) in the ischemic core and DWI lesion volume were assessed. Patients with and without HT following EVT were compared to evaluate the associated between a low ADC and HT.</p> Results <p>Seventy-eight patients were enrolled, 25 (32%) of whom experienced HT after EVT. Parenchymal hematomas were observed in 17 patients (22%). Statistically significant differences in peak ADC and onset-to-recanalization time were observed between patients with and without HT (<i>p</i> = 0.0036 and <i>p</i> = 0.0151, respectively). In multivariate analysis, a low peak ADC was associated with HT (odds ratio [OR], 0.839; <i>p</i> = 0.018), but onset-to-recanalization time was not. A threshold peak ADC resulting in HT was calculated as &lt; 480 × 10⁻⁶ mm²/s.</p> Conclusions <p>A low peak ADC was associated with HT after EVT for a large ischemic core. The threshold of the peak ADC that resulted in HT was &lt; 480 × 10⁻⁶ mm²/s. These results suggest that evaluating the ischemic core is necessary to confirm EVT.</p>