Background. Adolescents/young adults (AYAs) with acute lymphoblastic leukemia (ALL) are more likely to have chemotherapy-related complications than children. In addition, several reports have shown that infections account for most of the therapy-related mortality during cancer treatment in AYAs. Thus, we hypothesized that chemotherapy-induced myelosuppression is more severe in AYAs than in children, and the state of neutropenia was compared between children and AYAs using the D-index, a numerical value calculated from the duration and depth of neutropenia. Procedure. This study retrospectively analyzed 95 patients newly diagnosed with ALL at our institution between 2007 and 2019. Of these, 81 were children (< 15 years old) and 14 were AYAs (≥ 15 years old). The D-index and duration of neutropenia during induction chemotherapy for ALL were compared between children and AYAs. Results. The median D-index of children was significantly higher than that of AYAs (8,187 vs. 6,446, respectively, P = 0.017). Moreover, the median duration of neutropenia was also significantly longer in children than in AYAs (24.0 days vs. 11.5 days, respectively, P = 0.007). Conclusion. Contrary to our expectations, myelosuppressive toxicity during induction chemotherapy for ALL was more severe in children than in AYAs.