Engraftment syndrome (ES) after hematopoietic cell transplantation (HCT) is increasingly diagnosed. Common features include fever, pulmonary vascular leak, rash and organ dysfunction. Different diagnostic criteria likely account for the wide (7-90%) range of reported incidences. ES typically occurs within 4 days of granulocyte recovery although a recently described seemingly similar syndrome occurs >1 week before granulocyte recovery after umbilical cord blood cell transplants. Although the clinical manifestations of ES may be identical to those of acute GVHD, ES also has been well described in patients without acute GVHD. The data are conflicting as to whether ES is associated with a higher nonrelapse mortality and worse survival after HCT. The pathophysiology of ES is unclear, but endothelial injury and activated granulocytes in the setting of proinflammatory cytokines may be important. ES typically is self-limited, but, like acute GVHD, responds to corticosteroids. Because ES and acute GVHD may have overlapping features and response to therapy, these disease processes may often not be distinct events. Moreover, features of ES may overlap with those of drug- and radiation-induced toxicities and infection. Further research to better characterize the clinical spectrum and etiology of ES and to determine its relationship to GVHD is needed.