OBJECTIVE: Our aim was to expand research on predictors of health-related quality of life (HRQOL) for adolescent and young adult survivors of childhood brain tumors who are not living independently by evaluating the mediating role of family functioning in the association of disease severity/treatment late effects with survivor self-report and caregiver-proxy report of physical and emotional HRQOL. METHODS: Mothers (N = 186) and their survivors living at home (N = 126) completed self-report and caregiver-proxy report of physical and emotional HRQOL. Mothers completed family functioning measures of general family functioning, caregiving demands, and caregiver distress. Medical file review and caregiver report were used to evaluate disease severity/treatment late effects. RESULTS: Using structural equation models, family functioning was adjusted for sociodemographic factors. Disease severity/treatment late effects had significant direct effects on self-report and caregiver-proxy report of physical and emotional HRQOL. Family functioning had a significant direct effect on caregiver-proxy report of physical and emotional HRQOL, but these findings were not confirmed for self-report HRQOL. Model-fit indices suggested good fit of the models, but the mediation effect of family functioning was not supported. CONCLUSIONS: Disease severity/treatment late effects explained self-report and caregiver-proxy report of physical and emotional HRQOL for these adolescent and young adult survivors of childhood brain tumors. Family functioning was implicated as an important factor for caregiver-proxy report only. To enhance physical and emotional HRQOL, findings underscore the importance of coordinated, multidisciplinary follow-up care for the survivors who are not living independently and their families to address treatment late effects and support family management.