To assess the effect of the patient's posture (erect or supine) during rehabilitation sessions on pain, function, and the coracoclavicular (CC) distance after arthroscopic treatment of acromioclavicular joint dislocation. Sixty patients with acute type III or V acromioclavicular dislocation injuries were randomly allocated into 2 groups according to their posture during the rehabilitation phase: supine rehabilitation group (SRG) or erect rehabilitation group (ERG). Arthroscopic stabilization with a suspensory fixation device was used in all patients. The visual analog scale (VAS) score was assessed on the first postoperative day and at 1, 3, and 6 months postoperatively. The Constant-Murley score (CMS) was recorded preoperatively and at 3, 6, 12, and 24 months postoperatively. The CC distance was assessed preoperatively; on the first postoperative day; and at 6, 12, and 24 months postoperatively. No significant differences were found between the 2 groups in terms of the VAS score, CMS, and CC distance changes. A significant improvement over the follow-up phase was identified in the VAS score and CMS in both groups. The CC distance in both groups was significantly reduced from preoperatively (29.34 mm in the ERG and 28.65 mm in the SRG) to the first postoperative day (10.44 mm and 10.11 mm, respectively). However, a statistically significant re-widening of the CC distance (P < .001) occurred within the first 6 months after surgery (13.55 mm in the ERG and 13.50 mm in the SRG) and at 12 months (15.51 mm and 15.80 mm, respectively). The patient's posture during early postoperative rehabilitation does not affect the CC distance changes. Level I, randomized prospective comparative study.