The purpose of this study was to characterize a subgroup of cuff-deficient patients with isolated loss of active external rotation (ILER) but preserved active elevation and to evaluate the outcomes of the L'Episcopo procedure to restore horizontal muscle balance. During a 10-year period, 26 patients (14 men, 12 women) were identified with ILER in the setting of massive irreparable posterosuperior cuff tears. A modified L'Episcopo tendon transfer was performed to restore active external rotation and to improve shoulder function. The mean age at surgery was 64.5 years (29-83 years). Patients were evaluated with a mean follow-up of 52 months (range, 24-104 months). Preoperatively, despite maintained active elevation (average of 161°), ILER patients complained about loss of spatial control of the arm and difficulties with activities of daily living. On computed tomography scan or magnetic resonance imaging, there was severe fatty infiltration of infraspinatus and absent or atrophic teres minor. After L'Episcopo transfer, 84% of patients were satisfied. The gain in active external rotation was +26° in arm at the side and +18.5° in 90° abduction. Adjusted Constant score and Simple Shoulder Value increased from 63.6% to 86.9% and from 36.9% to 70.8%, respectively (P < .001). The ADLER score increased from 10 to 24.5 points (P < .002). Two patients with advanced cuff tear arthropathy (Hamada stage 3 and 4) required conversion to a reverse shoulder arthroplasty (RSA) 7 and 9 years after the index surgery. ILER is a distinct entity that is a cause of severe handicap because of loss of spatial control of the upper limb. This symptom is related to absent or atrophied infraspinatus and teres minor. In properly selected cases (Hamada stage 1 or 2), the modified L'Episcopo transfer is effective at restoring anterior-posterior rotator cuff force balance. In more advanced cuff tear arthropathy (Hamada stage ≥3), the tendon transfer should be performed with an RSA because of possible secondary degeneration of the glenohumeral joint.