Objective: To investigate the morphological changes of the roof of the subacromial bursa (SAB) and its involvement extent after rotator cuff tear. Methods: In the experimental group, the roof of SAB was obtained from 30 cases of rotator cuff tear both at the tear site and a site 2.5-3.0 cm distal to the tear site during rotator cuff repair. In the control group, the roof of SAB was obtained from the exposed site of recurrently dislocated shoulder or fractured humeral shaft of 8 cases.The specimens were stained with hematoxylin and eosin and observed under a transmission electron microscope. The cell number was quantitated through counting the bluestained nucleus in SAB with a computer image analysis system.Results: The number of cells increased significantly in the roof of SAB in the experimental group compared with that of the control group. However, no difference of the bursal reaction was found among the type of rotator cuff tear, the bursa thickness and the presence of fluid in the bursa. The great majority of cells were type B cells observed under the transmission electron microscope.Conclusions: The increase in cell number in the roof of SAB in the experimental group is a reactive increase rather than an inflammatory process and the involvement of SAB is not limited in extent. The change of the roof of SAB is a secondary reaction to the rotator cuff tear.