Results: In a cohort of 59 patients, the median age was 66 [55–81] years and SAPS II was 38 [24–48]. After RBCtransfusion, endothelium-dependent microvascular reactivity improved in 35 (59%) patients, but worsened in 24others (41%). Comparing clinical and biological markers revealed that baseline blood leucokyte counts distinguishedimproving from worsening patients (10.3 [5.7; 19.7] vs. 4.6 [2.1; 7.3] × 109/L; p = 0.001) and correlated with variationsof microvascular reactivity (r = 0.36, p = 0.005). Blood platelet count was also higher in improving patients (200 [97;280] vs 160 [40; 199] × 103/mL, p = 0.03) but did not correlate with variations of microvascular reactivity. Weobserved no intravascular hemolysis (HbCO, heme, bilirubin, LDH), but recorded a significant increase in RBCmicroparticle levels specific to improving patients after transfusion (292 [108; 531] vs. 53 [34; 99] MP/μL; p = 0.03).The improvement in microvascular dilation was positively correlated with RBC microparticle levels (R = 0.83, p <0.001) and conversion of arachidonic acid into vasodilating eicosanoids.