We hypothesized that the individual hematological response to chemo/ radiotherapy may be used as a parameter to assess the degree of myeloablation and probability of transplant-related events. This study included 58 pediatric patients who underwent allogeneic hematopoietic stem cell transplantation (HSCT). White blood cell (WBC) ratio (pre-conditioning WBC: day 0 WBC), day 0 WBC count, and WBC nadir day were used as potential indicators of myeloablation. The association between WBC kinetics and clinical result of HSCT was investigated. There was a positive correlation between WBC ratio and the date of engraftment. A positive correlation was noted between day 0 WBC count and engraftment day. There was a negative correlation between WBC nadir day and engraftment day. WBC nadir day was lower in patients with acute graft-versus-host disease (GVHD) than in cases without acute GVHD. Among patients who had fever during the conditioning regimen, the WBC ratio was higher, day 0 WBC count was lower, and WBC nadir day was lower in patients who developed >5 days of fever between day 0 and day +30. The present preliminary study suggests that WBC kinetics may be used as a measure of initial hematological response to the conditioning regimen and perhaps in determining the degree of myeloablation.