We studied the role of interleukin (IL)-8 during engraftment after hematopoietic stem cell transplantation. In 40 consecutive patients undergoing either allogeneic marrow (n=32) or autologous peripheral blood stem cell transplantation (n=8), IL-8 plasma levels were serially determined. Median IL-8 concentrations peaked during the neutropenic phase and, subsequently, subsided to pretransplant levels in patients achieving engraftment. In all patients, we observed an inverse correlation of IL-8 with leukocytes (P<0.0001) and a direct correlation of IL-8 with the extent of neutropenia (P<0.0001). Four patients who developed graft failure showed sustained median IL-8 concentrations of >300 pg/mL together with persistent neutropenia. This marked elevation of IL-8 was statistically significant as early as day 11 after transplantation, at a time when no other evidence alluded to imminent graft failure. Our data suggest that IL-8 may play an important role during engraftment after hematopoietic stem cell transplantation.