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Purpose Osteonecrosis (ON) is a serious complication of the treatment of childhood acute lymphoblastic leukemia (ALL); however, data relating to ON in Asian pediatric patients with ALL are scarce. Therefore, we performed a retrospective analysis of cohorts of Japanese patients with ALL to clarify the incidence, clinical characteristics, and risk factors of ON. Patients and Methods The incidence and characteristics of ON were determined in patients with ALL (n = 1,662) enrolled in two studies from the Japan Association of Childhood Leukemia Study (JACLS) group (n = 635 and n = 1,027 patients treated with the ALL-97 and ALL-02 protocols, respectively). Results In total, 24 of 1,662 patients suffered from ON, of which 12 of 635 and 12 of 1,027 patients were treated with the ALL-97 and the ALL-02 protocol, respectively. Of the 24 patients, 23 were older than 10 years. In multivariate analysis, age (≥ 10 years) was the sole significant risk factor for ON ( P < .001). Separate evaluation of patients ≥ 10 years of age indicated a 5-year cumulative incidence of ON of 7.2% (95% CI, 4.0% to 12.6%) and 5.9% (95% CI, 3.3% to 10.4%) in the ALL-97 and the ALL-02 protocol, respectively, which was lower than reported previously, despite an administration of dexamethasone (DEX) similar to that in comparable studies; however, concomitant administration of DEX and l-asparaginase was reduced in the JACLS protocols. Conclusion We identified a low frequency of ON in the JACLS ALL-97 and ALL-02 studies. Although the sole risk factor for ON was age (≥ 10 years), even among high-risk patients, ON incidence was significantly lower than that reported in previous studies. These results suggest that, not only the total amount of DEX, but also how DEX and l-asparaginase are administered, which affects the clearance of DEX, may be associated with ON incidence in patients with ALL.