Evidence-based literature recognizes that the different degrees of agreement between a child self-report and a proxy-report depend on the characteristics of the domains, the child's age and illness, the proxy's own perspective on QoL, and family attendance during the child's hospitalization. This study aims to determine the degree of agreement between proxy-reports and child self-reports on quality of life (QoL) for children with hematologic malignancy ranging in age from 5 to 18 years who are undergoing treatment.We retrieved clinical QoL data from a study titled "Dynamic change in QoL for Vietnamese children with hematologic malignancy" from April 2021 to December 2022. To evaluate the magnitude of agreement between self-reports and proxy-reports, intraclass correlation coefficients (ICCs) for 259 pairs of measurements were quantified. Using independent t tests, the mean differences between self-reports and proxy-reports were tested.Moderate agreement was consistent through all age groups for five subscales, including physical, psychosocial, pain, nausea, and procedural anxiety (ICCs ranged from 0.53 to 0.74). The weakest agreement appeared in two groups, subjects aged 5-7 years and 13-18 years on six domains (school, treatment anxiety, worry, cognitive problems, perceived physical appearance, and communication) (-0.01 to 0.49). Child self-rating was consistently higher than that of proxies for the physical, emotional, and nausea domains among children aged 5-7 years and for procedural anxiety, treatment anxiety, and cognitive problems among children aged 8-12 years. Conclusion: The agreement level of self-reports and proxy-reports was differently distributed by child age and the PedsQL domains. The proxy children agreement on QoL among children with hematologic malignancy was divergent according to the different age groups, which could potentially be explained by proxy-child bonding at different stages of childhood development. Our recommendation for future studies is to explore children's age as a potential factor influencing proxy agreement on QoL among children with cancer. What is Known: • Children and their proxies may think differently about quality of life (QoL). • Comparing two sources of data (i.e., child and proxy) on aspects of QoL can help identify the discrepancies between children's perceptions of their QoL and their parents' perceptions. This can be useful in terms of identifying potential areas for improvement or concern and may also be helpful in making decisions about treatment and care. What is New: • Our study results demonstrated that proxies who comprised children aged 5-7 years or 13-18 years reported differently among domains that cannot be expressed verbally or with body language, including cognitive problems, perceived physical appearance, and communication. • Children generally perceived their QoL to be better than their proxies. Therefore, a more comprehensive understanding of children's QoL may require the consideration of multiple sources of data from various perspectives.