In patients with arthritis, synovial tissue is easily accessible for analysis. Blind needle biopsy is a simple and safe procedure. Arthroscopic biopsy is also safe, it allows access to most sites in the joint and it can provide adequate tissue for extensive laboratory investigations, both before and after successful therapy. Synovial tissue analysis has been successfully applied to the study of disease mechanisms and response to treatment. In addition, there may be an indication for diagnostic synovial biopsy in selected cases.