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Background and purpose Magnetic resonance imaging quantitative T2* mapping has shown reliable identification of thrombus red blood cell content in vitro. The thrombus composition has been in vivo, associated with outcomes after endovascular therapy for acute ischemic stroke. We aim to analyze the red blood cell content of thrombi retrieved from patients with large vessel occlusions in relation to the thrombus-T2* relaxation time in magnetic resonance imaging. Material and methods Consecutive acute ischemic stroke patients treated by endovascular therapy were scanned with an magnetic resonance imaging quantitative T2* mapping sequence. Quantitative histologic evaluations of red blood cell content were performed. A linear regression assessed the association between vascular risk factors, comorbidities, antithrombotic drugs intake, baseline National Institutes of Health Stroke Scale (NIHSS), intravenous thrombolysis before endovascular therapy, time between onset and groin puncture, patient's outcome at 3 months, magnetic resonance imaging quantitative T2* mapping results, and the red blood cell content of thrombi. The correlation between the mean thrombus-T2* relaxation time and red blood cell content was assessed by calculating the Pearson correlation coefficient. Results Among 31 thrombi, 16 were “Fibrin rich” and 15 “red blood cell dominant.” The median red blood cell content was 39 (range, 0–90; interquartile range, 37). The median (interquartile range) thrombus-T2* relaxation time was shorter in “red blood cell dominant” thrombi (21, interquartile range 6) than in “Fibrin rich” thrombi (24, interquartile range 7), without significant difference ( p = 0.15), as shown in the Box plot. An inverse correlation between thrombus-T2* relaxation time and red blood cell content was found, with a correlation coefficient of −0.41 (95% CI, −0.67 to −0.08, p = 0.02). Conclusion Our study shows that a shorter thrombus-T2* relaxation time is related to a higher red blood cell content within in vivo thrombi.