Intravascular haemolysis and nitric oxide-dependent endothelial dysfunction are important features of severe falciparum malaria but have not been evaluated in knowlesi malaria. In Malaysian adults with knowlesi malaria, cell-free haemoglobin (CFHb) is increased in proportion to disease severity, and is independently associated with parasitaemia, and the Weibel-Palade body (WPB) constituents angiopoietin-2 and osteoprotegerin (OPG). As previously shown with angiopoietin-2, OPG is increased in proportion to disease severity, and is independently associated with creatinine, lactate, interleukin-6, endothelial cell adhesion molecules ICAM-1 and E-selectin, and impaired microvascular reactivity. OPG is also independently associated with nitric oxide-dependent endothelial dysfunction. Angiopoietin-2 and OPG are both independent risk factors for acute kidney injury. These findings suggest that haemolysis-mediated endothelial activation and release of WPB constituents is likely a key contributor to end-organ dysfunction, including AKI, in severe knowlesi malaria.