Background: It is necessary to better understand the structural characteristics of the supraspinatus tendon and associated muscle after rotator cuff repair and in the event of retear. Purpose: To study structural differences between the repaired and contralateral shoulders 1 year after rotator cuff repair in patients who received either progressive exercise therapy (PR) or usual care (UC) in a randomized controlled trial and to investigate whether there was interaction with tendon retear and limb dominance. Study Design: Cohort study; Level of evidence, 2. Methods: Patients with surgically repaired traumatic full-thickness rotator cuff tears involving the supraspinatus tendon were included. After surgery, they were randomized to PR or UC (active from postoperative week 2 or 6, respectively). The subacromial structures (acromiohumeral distance, supraspinatus tendon thickness, and vascularity) and the supraspinatus muscle thickness were examined with ultrasound at the 1-year follow-up. Results: A total of 79 patients were included. The characteristics of the 2 intervention groups (PR and UC) were comparable, including the Western Ontario Rotator Cuff Index score and number of retears. The authors found significantly thinner supraspinatus tendon (PR, P <.001; UC, P =.003) and reduced acromiohumeral distance (PR, P =.023; UC, P =.025) in the repaired versus the contralateral shoulders in both intervention groups. For neovascularization, there was no interlimb difference in either of the groups or between groups (PR vs UC). In patients with intact tendons, there was no interlimb difference in the muscle thickness, but in patients with tendon retear the muscle was significantly thinner on the repaired side (P =.024 and P <.001, respectively). When the dominant supraspinatus tendon was repaired (both groups), it was significantly thinner than the nondominant healthy tendon, but this difference was not seen when the nondominant supraspinatus tendon was repaired (P =.006). Conclusion: One year after rotator cuff surgery, the repaired supraspinatus tendon was significantly thinner and the corresponding acromiohumeral distance was reduced. In patients with retear, the supraspinatus muscle was significantly thinner on the repaired side and early initiation of tendon-loading exercises did not affect these findings. Registration: NCT02969135 (ClinicalTrials.gov identifier).