The clinical findings of fever and skin rash with or without evidence of fluid retention, which mimic engraftment syndrome, have been observed during the pre-engraftment period in patients undergoing hematopoietic stem cell transplantation. In order to characterize this newly observed clinical syndrome called pre-engraftment syndrome (pES), we retrospectively analyzed the clinical records of 50 patients. Three out of 14 patients (23.1%) who underwent cord blood stem cell transplantation developed non-infectious fever, skin rash, and tachypnea 4-15 days prior to neutrophil engraftment. Two patients spontaneously recovered with fluid restriction and oxygen inhalation. One patient died of a complicated pulmonary hemorrhage in spite of aggressive supportive therapy and steroid treatment. Four out of 23 patients (17.4%) who underwent allogeneic bone marrow transplantation developed non-infectious fever and skin rash 4 to 5 days prior to neutrophil engraftment. All four of these patients recovered with only steroid treatment. These characteristic findings were not observed in patients who had undergone autologous peripheral blood stem cell transplantation. Interestingly, the speed of neutrophil engraftment was significantly faster for the patients suffering from pre-engraftment syndrome. The close observation and further pathophysiological research are required to better understand this syndrome.