Biopsy specimens of the synovial membrane were obtained during arthroscopy and surgical meniscectomy from the knees of 20 patients with meniscus lesions. The aim of this study was to identify the morphological and immunological changes which appear in the synovium during the earliest phase of osteoarthrosis. Interstitial deposits of IgG and, in some cases, C3 were found not only in patients with evident arthrotic degeneration (cartilaginous lesions and synovitis), but also in patients who showed no overt arthrotic changes. Furthermore, light and electron microscopy showed an increased number of mast-cells with peculiar semilunar or piecemeal aspects of their secretory granules. These ultrastructural modifications are characteristic of slow, chronic release of mediators in response to a constant, moderate degranulatory stimulus. Our findings suggest that synovial changes may occur before the advent of cartilage degeneration and that the latter may be directly influenced by early pathological changes in the synovial membrane.