To study the superior-inferior stabilizing functions of the coracohumeral ligament (CHL) and the rotator interval capsule (RIC) with use of a material testing machine.The axial translations of the humerus with the superior-inferior translation force of 30 N applied were recorded under the following joint capsule conditions: (1) intact, (2) vented, (3) the CHL sectioned, and (4) the RIC incised in six cadaver shoulders. The order of sectioning was changed for conditions 3 and 4 in six other cadaver shoulders.With the arm in internal and neutral rotations, venting the capsule significantly increased the superior-inferior translation, which was unaffected by further sectioning of the CHL and the RIC. With the arm in external rotation, only the CHL contributed significantly to inferior stability, whereas both this ligament and the RIC contributed to superior stability to a lesser degree.The CHL is a stabilizer in superior inferior directions with the arm in external rotation, and the intra-articular pressure that is maintained by the intact RIC is a stabilizer in superior-inferior directions with the arm in internal and neutral rotations. These findings may provide a scientific background to support closure of the interval space to stabilize the shoulder and may explain part of the superior instability observed in shoulders with rotator cuff tears.