Background Children are at risk for behavioral and adaptive difficulties following pediatric brain tumor. This study explored whether familial/demographic, developmental, diagnostic, or treatment‐related variables best predict posttreatment behavioral and adaptive functioning. Methods Participants included 40 children (mean age = 12.76 years, SD = 4.01) posttreatment (mean time since diagnosis = 1.99 years, SD = 0.21) for pediatric brain tumor. Parents rated children's behavioral adjustment and adaptive functioning and provided demographic and developmental histories. Diagnostic and treatment‐related information was abstracted from medical records. Results Ratings of adaptive and behavioral functioning approximately 2 years postdiagnosis were within the average range, although the percentage of children exceeding clinical cutoffs for impairment in adaptive skills exceeded expectation, particularly practical skills. Premorbid behavior problems and tumor size predicted posttreatment adaptive functioning. After accounting for adaptive functioning near diagnosis, premorbid behavior problems predicted declines in adaptive functioning 2 years postdiagnosis. After accounting for adjustment near diagnosis, no variables predicted declines in behavioral adjustment. Conclusions Children may be vulnerable to reduced adaptive functioning following pediatric brain tumor treatment, especially in practical skills. Assessing prediagnosis functioning and diagnostic and treatment‐related variables may improve our ability to predict those at greatest risk, although those factors may be less helpful in identifying children likely to develop behavioral difficulties. Screening of these factors in tertiary care and long‐term follow‐up settings may improve identification of those at greatest need for support services.