Subacromial bursal tissue was studied in 12 patients operated on for painful (10 patients with constant pain and 2 patients with pain on motion) rotator cuff tendinitis/impingement syndrome. The Neer acromioplasty technique was used. Six patients had moderate inflammatory changes and one had a slight inflammation. In three of the five remaining patients, the subacromial bursa did not show any signs of inflammatory involvement, but patients experienced pain at rest and at night, reflecting clinical inflammation in tissues other than the bursa. The two patients with pain only on strain did not show inflammation of the bursa. Immunohistochemical typing of the bursal tissue disclosed a typical chronic mononuclear cell infiltrate consisting mainly of CD2-positive T lymphocytes (50-80% of all inflammatory cells), accompanied by less frequent CD11b (C3bi receptor)-positive monocyte/macrophages (10-40%). The relative paucity of plasmablasts/plasma cells expressing PCA-1 suggests this to be an inflammatory rather than an immune response. Active involvement of some of the local cells is suggested to be the source of algogenic and hyperalgesic substances contributing to pain in chronic shoulder pain syndromes.