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Health-related quality of life measures for the adolescent patellar instability population are limited. The Banff Patella Instability Instrument (BPII) is a disease-specific health-related quality of life that has been tested on a mixed adult and adolescent population but has not been exclusively tested with an adolescent population. The purpose of this study was to determine the validity, reliability, and responsiveness of the BPII within an adolescent population.Content validation was tested with 3 measures: floor and ceiling effects; the Cronbach alpha; readability grade level. Reliability was measured with an intraclass correlation coefficient (2, k) and the SEM. Responsiveness was measured using a 1-way analysis of variance comparing preoperative BPII scores to postoperative BPII scores at 6, 12, and 24 months. Eta squared was calculated to measure effect size.The BPII demonstrated no floor or ceiling effects. The Cronbach alpha for the BPII ranged from 0.97 to 0.98 for the preoperative and postoperative time periods. The readability index was a mean grade level of 8.9. Test-retest reliability assessment demonstrated an intraclass correlation coefficient (2, k) of 0.99 indicating strong reliability. The SEM was 1.3. There was a statistically significant difference in BPII scores for the 4 different time interval measures (F3, 176=50.04; P<0.001). Eta squared was 0.46 demonstrating a moderate to large effect.The BPII demonstrated validity, reliability, and responsiveness in an adolescent population. Furthermore, it has demonstrated an appropriate cognitive reading grade level of 8.9, commensurate with the mean age (16.08 y) of the population tested.Level III.