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Introduction and Hypothesis: Multiple studies have demonstrated no statistically significant association between time after stroke onset and initial infarct volume. Factors other than time may play a role in infarct growth. We sought to investigate association between collateral status and infarct growth rate in acute ischemic stroke (AIS) patients.Methods: We included 130 consecutive patients with CTA showing ICA and/or proximal MCA occlusions who had DWI within 8 hours of stroke onset. Collateral status was categorized into three groups: poor (none or minimal), intermediate (present but < contralateral side) and good (≥ contralateral side). DWI lesion volumes were measured and infarct growth rate was calculated using MRI time after stroke onset. Mann-Whitney test and correlation coefficient were used for statistical analysis.Results: In our 130 patients, 62 female (48%), the average values (mean±SD) were: age 70 ± 17 years, NIHSS 16 ± 6, DWI volume 59 ± 65 mL, time after stroke onset 4 ± 2 hours, and infarct growth rate 17 ± 23 mL/hour. 19 (14.6%) had poor, 75 (57.7%) had intermediate, and 36 (27.7%) had good collaterals. Infarct growth rate and DWI lesion volume were significantly increased with decreased collateral quality (p<0.0004 for all group comparisons). Patients with good collaterals were younger (p=0.004 and p=0.018 compared to poor and intermediate groups respectively) and had lower NIHSS scores (p< 0.001). Time after stroke onset, gender, or occlusion site (ICA vs MCA) were not significantly different among different collateral groups. There was no correlation between time and DWI volume (r2=0.02, p=0.8) or collateral status (r2=0.05, p=0.6). There was significant correlation between collateral status and infarct growth (r2=-0.6,p<0.0001).Conclusion: AIS patients with good collaterals have small initial DWI lesion volumes and slower infarct growth rates. These patients may be candidates for treatment options outside traditional time windows.