Acute graft-versus-host disease (aGVHD) is a potentially life-threatening complication after allogeneic stem cell transplantation (alloSCT) commonly treated with steroids. Steroid-refractory aGVHD is associated with thrombotic microangiopathy (TAM), supporting the notion of endothelial dysfunction as a risk factor for poor prognosis. Several diagnostic criteria exist to diagnose TAM; however, emerging data points to the clinical importance of the endothelial activation and stress index (EASIX) as biomarker in the context of alloSCT. The EASIX integrates three basic lab parameters (serum lactate dehydrogenase and creatinine concentrations with platelet counts) into one ratio that produces continuous values to allow monitoring of impending endothelial complications after alloSCT. This chapter provides an overview of the use of EASIX as prognostic marker for mortality after aGVHD and endothelial complications.