There is no financial information to disclose. During office visits, joint ranges of motion are often measured by surgeons using a visual estimate or therapists using a goniometer. This study aimed to assess the accuracy of these measures, and the hypotheses were: (1) poor agreement and (2) poor precision between humerothoracic external rotation (HTER) and glenohumeral cross-body adduction (GHCBA) when measured by a surgeon’s clinical estimate, an occupational therapist’s goniometer, and a motion capture system’s measurements in children with brachial plexus birth palsy (BPBP), and (3) poor correlation between HTER measured by motion capture and external rotation (ER) Mallet classification score. Twenty-six children (9.9±3.2 years) with BPBP participated in this study. A pediatric hand surgeon visually estimated each child’s passive HTER and passive GHCBA during the patient’s clinical visit. An occupational therapist measured the same parameters using a goniometer while motion capture (Vicon, Oxford, UK) measurements of thorax, scapula, and humerus orientations were simultaneously collected. One-way ANOVAs were completed to test for differences between measurement techniques, Bland-Altman plots were created to look at precision and agreement across measurements, and Spearman rho correlation was used exam the relationship between motion capture and Mallet Scale scores. Comparison of clinical estimates, goniometer measurements and motion capture measurements demonstrated no significant differences (P = 0.751) for GHCBA, while all HTER measures were significantly different from each other (P < 0.02). The GHCBA comparisons exhibited good accuracy with the Bland-Altman plots with the mean difference near zero, but poor precision with a minimum of 20° deviations from neutral (Fig. 73-1). For HTER, there was poor precision and accuracy (Fig. 73-1). Considering precision using motion capture as the gold standard, both goniometer measurements and clinical estimates overestimated HTER by 9° and 24° respectively (mean differences). There was also poor accuracy, with the smallest 95% CI across all comparisons being + 24°. External rotation position Mallet scores correlated with surgeon estimates of HTER (r = -0.43, P = 0.02), but not motion capture (r = -0.24, P = 0.229). •Compared to the gold standard motion capture measurements, the surgeon’s clinical estimates and occupational therapist’s goniometer measurements had both poor precision and accuracy for HTER and poor precision for GHCBA.•Surgeon-estimated HTER and Mallet ER scores were poorly correlated, which calls into questions the accuracy of Mallet score measurements.•Motion capture is an important tool with improved accuracy and precision of shoulder motion measurements and should be considered for routine clinical care for patients with BPBP.