Many different models exist that try to explain the pathogenesis of rheumatoid arthritis. However the perpetuation of synovitis in rheumatoid arthritis likely results from a complicated interplay among lymphocytes, fibroblast-like cells and macrophage-like cells. Unraveling and understanding the highly complex inter-relationship between these cells causing the initiation and perpetuation of rheumatoid arthritis would be very helpful in finding successful therapeutic approaches. In this study we analysed the potential pathophysiologic role of perforin-positive cells in the destructive proccesses of rheumatoid arthritis. These cells, alone or in combination with other approaches, could offer a novel therapeutic target for interfering with the chronic inflammatory proccesses. Objective: To obtain insight into the potential role of the cytolytic action mediated by perforin in the course of rheumatoid arthritis, we studied the immunophenotypic characteristics of lymphocytes containing perforin simultaneously in peripheral blood (systemic level) and in synovial fluid and synovial membrane (local level) in patients in acute or chronic phase of rheumatoid arthritis. As controls served cells from patients suffering of osteoarthrosis. Methods: Our procedure for simultaneous detection of intracellular (perforin) and cell surface antigens by flow cytometry was used. Mean fluorescence intensity (MFI) was a measure of the mean perforin content per cell. To determine the localization of perforin positive cells immunohistochemical staining methods of frozen sections were used. Results: Significantly higher percentages of perforin positive cells as well as relative numbers of perforin molecules per cell, were found at both the systemic and local level in the acute phase of rheumatoid arthritis in comparison with other groups. Perforin expression in acute rheumatoid arthritis was higher at the systemic level (peripheral blood) in comparison with local level (synovial membrane and synovial fluid), but the relative increase of perforin positive cells was much higher at the local level. Conclusions: Perforin positive cells participate in the acute phase of rheumatoid arthritis, maintaining and perpetuating the immunological events, and probably contributing to the tissue destruction.