The role of rotator interval in shoulder pathology and the effect of its closure are not well understood. In addition, the effect of rotator interval closure on intra-articular glenohumeral volume (GHV) remains unknown. We conducted a study to quantify the GHV reduction obtained with an arthroscopic rotator interval closure and to determine whether medial and lateral interval closures resulted in different degrees of volume reduction. We dissected 8 fresh-frozen cadaveric shoulders (mean age, 64.4 years) to the level of the rotator cuff. Volumetric measurements were taken before and after medial and lateral rotator interval closure incorporating the superior glenohumeral ligament and the upper portion of the subscapularis. Arthroscopic closure of the rotator interval with 2 sutures reduced GHV by a mean of 45%. More volume reduction resulted with use of a single lateral interval closure stitch than with use of a single medial stitch (35% vs 24%; P < .02). Arthroscopic rotator interval closure with 2 plication stitches is a powerful tool in reducing intracapsular volume of the shoulder and may be a useful adjunct in restoring glenohumeral stability. If a single plication stitch is preferred, a lateral stitch (vs a medial stitch) can be used for a significantly larger reduction in shoulder volume.