OBJECTIVE: This article examined associations between change in youth and family characteristics during youth anxiety treatment and long-term anxiety severity and overall functioning. METHOD: Participants (N = 488; age 7-17 years; 45% male; 82% white) were randomized to 12 weeks of cognitive behavioral therapy (Coping Cat), medication (sertraline), their combination, or pill placebo in the Child/Adolescent Anxiety Multimodal Study (CAMS). A subset participated in the naturalistic follow-up Child/Adolescent Anxiety Multimodal Extended Long-term Study (CAMELS; n = 319; 3.70-11.83 years post-treatment). The current secondary analyses examined how change in anxiety severity (Child Global Impression-Severity), overall functioning (Children's Global Assessment Scale), caregiver psychopathology (Brief Symptom Inventory), caregiver strain (Family Burden Assessment Scale), and family dysfunction (Brief Family Assessment Measure) during CAMS was associated with anxiety severity and overall functioning years later (M = 7.72 years). CAMS procedures were registered on clinialtrials.gov. RESULTS: Improvements in factors related to functioning (i.e., overall functioning, family dysfunction, caregiver strain) were associated with improvements in anxiety severity in CAMELS (|βys| ≥ .04, ps ≤ .04). Improvements in factors related to psychopathology (i.e., anxiety severity, caregiver psychopathology) were associated with improvements in overall functioning in CAMELS (|βys| ≥ .23, ps ≤ .04). It was changes in each of the variables examined (rather than baseline values) that predicted anxiety severity and overall functioning. CONCLUSIONS: Both youth and family factors play a significant role in long-term treatment outcomes. Therapists would be wise to monitor how these factors change throughout treatment. (PsycInfo Database Record (c) 2021 APA, all rights reserved).