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The change in the cross-sectional area of a repaired muscle, measured with use of magnetic resonance imaging (MRI), could be an indicator of recovery of muscle function. The aims of this study were to evaluate the change in the area of the supraspinatus muscle between the immediately postoperative and one-year postoperative MRIs and to identify factors associated with the change.Eighty-eight patients with a full-thickness rotator cuff tear were included. MRI was performed three days and one year after surgery. Patients were classified into two groups according to whether the area of the supraspinatus increased or decreased between these two time points. Outcomes including pain, shoulder motion, strength, and commonly used clinical scores were assessed preoperatively and at three, six, and twelve months after surgery. Changes in the rotator cuff muscles and retear of the repaired tendon were also evaluated.The area of the supraspinatus muscle increased in twenty-nine (33%) of the patients and decreased in fifty-nine (67%). The change in area was 36.75 ± 27.94 mm2 in the group in which it increased and −94.25 ± 70.38 mm2 in the group in which it decreased (p < 0.001). Multiple regression analysis indicated that a lower preoperative Simple Shoulder Test (SST) score, better gross visual grade of the tendon at surgery, and greater strength of the supraspinatus at six months postoperatively were associated with an increase in the area. No retear or Sugaya grade of 3 was found in any patient in whom the area increased, whereas 34% of the patients in whom the area decreased had a retear (p < 0.001).This study showed that the cross-sectional area of the supraspinatus muscle could either increase or decrease during the first year after rotator cuff repair and that robust healing (indicated by a Sugaya grade of 1 or 2) and good tendon quality at surgery were important factors associated with an increase in the area.Prognostic Level II. See Instructions for Authors for a complete description of levels of evidence.