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Background: The chemotherapy regimens recommended for both rhabdomyosarcoma (RMS) and Ewing sarcoma (ES) patients are myelosuppressive and can reduce the absolute neutrophil count (ANC) and subsequently increase the risk of febrile neutropenia (FN). However, only a few studies have focused on the efficacy and safety of granulocyte-colony stimulating factor (G-CSF) drugs in pediatric and adolescent patients with RMS and ES. Our objective was to investigate the efficacy and safety of mecapegfilgrastim, a biosimilar of pegfilgrastim, in prophylaxis of FN for pediatric and adolescent patients with RMS or ES., Methods: In this single-arm, single-center, prospective study, pediatric and adolescent patients with RMS or ES were enrolled to receive either VAC (vincristine, cyclophosphamide, dactinomycin) regimen or VDC (vincristine, cyclophosphamide, doxorubicin) regimen in a 3-week cycle, followed by treatment with mecapegfilgrastim (100 μg/kg, maximum 6 mg) given at 24 h after completing chemotherapy. The primary endpoint was the incidence rate of FN. Secondary endpoints included the incidence rate of grade 4 neutropenia, duration of ANC ≤ 0.5 × 10 9 /L, incidence rate of chemotherapy delay or reduction, use of antibiotics, and safety profile., Results: In total, 2 of the 30 (6.7%, 95% CI: 0.82-22.07) patients experienced FN after the first cycle of chemotherapy. Eight (26.7%, 95% CI: 12.28-45.89) patients experienced grade 4 neutropenia after receiving prophylactic mecapegfilgrastim. Eight patients experienced ANC ≤ 0.5 × 10 9 /L with a median duration of 4.5 days; among them, 6 patients reached the lowest point of their ANC level on day 7, and 5 of them recovered by day 10. No dose reductions, delays, or discontinuation of chemotherapy was reported. Twenty-one (70.0%) patients received antibiotics during the treatment period. No patient experienced FN in the 0-5 years and the 13-18 years groups, and 2 patients experienced FN in the 6-12 years group. Two patients, 6 patients, and no patient experienced grade 4 neutropenia in the 0-5 years, 6-12 years, and 13-18 years groups, respectively., Conclusion: Mecapegfilgrastim showed acceptable efficacy and safety profile in pediatric and adolescent patients with RMS or ES. Further randomized studies with large sample size are warranted., Trial Registration: This clinical trial was registered at Chictr.org.cn (No.ChiCTR1900022249). Registered on March 31, 2019., (© 2024. The Author(s).)