Background: The presence of poorly controlled symptoms negatively impacts the quality of life (QoL) throughout cancer treatment. The purpose of this multisite study was to explore the relationship between QoL and symptom adverse events (AEs) in children with advanced cancer over 6 months. Method: A prospective and longitudinal descriptive study design was used to collect QoL and symptom AE data from children aged 2 to 18 with advanced cancer. QoL was measured using the Pediatric Quality of Life Inventory (PedsQLTM) Cancer Module 3.0 and symptom AEs were measured using the Pediatric Patient-Reported Outcome-Common Terminology Criteria for AEs (PRO-CTCAEs®). Descriptive statistics were used to describe QoL and symptom AE data. Correlational analyses and generalized linear mixed models were used to examine the relationship between symptom AEs and QoL. Results: Forty-nine children participated in the study. The mean total PedsQLTM score was 73.86 for the sample across all time points. Children diagnosed with a central nervous system (CNS) tumor reported poorer QoL compared to children diagnosed with a hematologic malignancy or non-CNS solid tumor. Symptom frequency AEs of anxiety, pain, nausea, insomnia, hot flashes, and fatigue severity demonstrated the strongest and most significant negative correlation with total QoL scores. Analyses of the relationship between QoL and symptom AEs over time revealed time-specific significant differences with children who experienced frequency AEs of nausea, and anxiety reporting poorer QoL at time point 4 (week 8). Discussion: The Ped PRO-CTCAE® and PedsQLTM can be used to evaluate the relationship between symptom AEs and QoL in practice and in future research.