Synovitis in the subacromial bursa (SAB) and the gle-nohumeral joint (GHJ) is often seen in rotator cuff diseases. In order to clarify its significance, following studies were conducted. The mRNA expression levels of IL-1B, sIL-1ra and icIL-1ra and the amount of substance P in the SAB synovium were correlated with the degree of shoulder pain. The cytokine-mRNAs in the GHJ synovium expressed more significantly in full-thickness tears (perforating tears) than in non-perforating tears. Biochemical markers (MMP-1, MMP-3) in the GHJ fluid were significantly higher in massive cuff tears than in smaller tears. These findings suggest the possibility that SAB and GHJ synovitis in rotator cuff diseases are associated with shoulder pain and the development of glenohumeral arthropathy, respectively.