Children with brachial plexus birth palsy (BPBP) may undergo shoulder external rotation tendon transfers (ERTT) to improve function. In terms of outcome measurements, and according to the World Health Organization International Classification of Functioning, Disability, and Health model, ERTT reduces impairment as measured by range of motion (ROM), but has not been shown to improve activity and participation. Furthermore, correlation between these outcome measures has not been established. The Pediatric Outcomes Data Collection Instrument (PODCI) is a well-validated musculoskeletal health questionnaire that addresses activity and participation components of function. The aim of this study is to determine whether PODCI scores improve after ERTT, and whether this improvement correlates with improvement in ROM. A total of 23 children with a mean age of 6.3 years (range, 4.4-12.8 years) with BPBP and standard indications for ERTT underwent preoperative and 1-year postoperative shoulder ROM measurements, and parental completion of the PODCI. Change in ROM was compared with change in PODCI scores to determine if these were correlated. Average range of active shoulder abduction improved 35 degrees (P < 0.001), and average range of active external rotation improved 41 degrees (P < 0.001). The PODCI scores for Upper Extremity Function, Sports Function, and Global Function improved (12 points [P < 0.001], 4 points [P = 0.04], and 6 points [P = 0.001], respectively). Improvement in ROM did not correlate with improvement in PODCI scores. However, postoperative peak active abduction correlated strongly with postoperative PODCI scores for Upper Extremity Function, and Global Function (rs = 0.712 [P < 0.001], rs = 0.735 [P < 0.001], respectively), and moderately with Transfers and Basic Mobility and Sports Function scores (rs = 0.496 [P=0.016], rs = 0.449 [P = 0.032], respectively). For children with BPBP, ERTT is associated with reduced impairment and improved activity and participation. Maximum postoperative abduction is positively associated with PODCI scores, but change in ROM is not. Further study is needed to determine if ceiling effects or other factors account for the lack of correlation between these outcome measures. Level of evidence IV, case series.