Rheumatoid arthritis (RA) is a chronic inflammatory disease with primary manifestations in the synovial membrane. Tissue infiltrates are composed of T cells, B cells, and macrophages, but histopathological appearances vary widely and are rarely pathognomonic. Mechanisms underlying the phenotypic heterogeneity of rheumatoid synovitis are not known. To explore whether a correlation exists between the microscopic patterns of rheumatoid synovitis and in situ production of cytokines, tissue samples from 21 consecutive patients with clinically active RA were examined. Based upon the organization of the lymphocyte infiltrate, the synovial biopsies were categorized into three distinct subsets. Ten samples were characterized by diffuse lymphoid infiltrates without further microarrangement. In seven samples, lymphoid follicles with germinal center formation were detected, and in four specimens, granuloma formation was identified. In all specimens, cytokine transcription of interferon (IFN)-gamma, interleukin (IL)-4, IL-1 beta, tumor necrosis factor (TNF)-alpha, IL-10, and transforming growth factor-beta 1 was semiquantified with polymerase chain reaction and liquid phase hybridization. Each of the morphologically defined variants of synovitis displayed a unique cytokine profile. Low-level transcription of IFN-gamma, IL-4, IL-1 beta, and TNF-alpha was typical of diffuse synovitis. In follicular synovitis, IFN-gamma was the dominant cytokine, IL-4 was virtually undetectable, and IL-10 was abundant. Granulomatous synovitis demonstrated high transcription of IFN-gamma, IL-4, IL-1 beta, and TNF-alpha and could be clearly distinguished from the other phenotypes. To investigate whether differences in the synovial lesions were related to host factors, patients were compared for clinical parameters. Diffuse synovitis was seen in most of the patients with seronegative RA, the mildest form of the disease. In contrast, extra-articular spreading of RA with nodule formation was typically associated with granulomatous synovitis. In summary, RA patients display reproducible patterns in the organization and activity of synovial infiltrates. The correlation of microanatomy with tissue cytokine production suggests that several pathomechanisms can modulate the expression of the immune response in the synovial membrane.