Distinct myeloid DC and lymphoid DC subsets have been described, which regulate the nature and magnitude of immune responses. Therefore DC function must be carefully regulated, otherwise inappropriate responses may result in such chronic inflammatory diseases as rheumatoid arthritis (RA). In this study the composition and activation state of DC subsets was compared between autologous blood, synovial fluid and synovial tissue of RA patients using 4-colour flow cytometry. Preliminary results indicated that RA blood and normal blood had a similar ratio of DC subsets, both of which exist in a relatively inactivated state. In contrast, myeloid DC were predominant in RA synovial fluid and synovial tissue. In synovial tissue these myeloid DC were more highly activated and localized to lymphoid aggregates. Lymphoid DC were scarce in both synovial fluid and synovial tissue.