Quantifying changes in shoulder external rotation range of motion and strength after lower trapezius transfer in children with obstetric brachial plexus palsy (OBPP). Methods. This prospective study included five children with sequelae of OBPP, with a mean age of 6.4 years (range: 4–12 years) who underwent lower trapezius tendon transfer to restore active external rotation (ER) of the shoulder. Pre-operatively and at a 12-month follow-up assessment, we analyzed the passive and active shoulder ER, the modified Mallet score, and the shoulder rotator muscles strength using an isokinetic device. Results. Clinical parameters improved significantly after trapezius transfer pre-operative passive ER from −8° (range: −20–0°) to 37° (range: 15–50°) (p = 0.035), the modified Mallet score from 13 (range: 10–15) to 18 (range: 17–19) (p = 0.035). Strength testing revealed improved ER muscle strength in all five cases; the mean Peak Torque increased from 1.95 to 4.46 N·m, albeit non-significantly (p = 0.062). Two patients exhibited a winged scapula post-operatively. Conclusion. Lower trapezius transfer seems encouraging to restore shoulder external rotation in OBPP children but with non-significant strength improvement.