We showed that RBC transfusion remodels the microvascularreactivity in ICU patients and microvasculardilation improved in some patients, but worsened inothers. We report that circulating leukocyte count positivelycorrelates with microvascular reactivity after transfusion.The transfusion of one packed RBC unit was notassociated with significant intravascular hemolysis, but itcorrelated with increased levels of RBC microparticlesand modifications in the plasma lipidome in improvingpatients only. Larger studies are now required to confirmour results and unravel the clinical significance ofmicrovascular alterations and the putative, vasculo-protectiveeffects of transfusion-induced microparticles