Our objective was to investigate the clinical significance of Th1 and Th2-type cytokines, such as interferon-gamma (IFNgamma) interleukin (IL)-12, IL-4 and IL-10, in the mononuclear cells (MNC) of the synovial fluid (SF) in patients with rheumatoid arthritis (RA). The cytokine production in the MNC obtained from the SF (SF-MNC) in 30 patients with RA and 10 with gout was examined by measuring the mRNA levels of IFNgamma, IL-12, IL-4 and IL-10 by semiquantitative RT-PCR. The mRNA levels of IFNgamma, IL-4 and IL-10 were significantly higher in the SF-MNC of RA patients than in those of gout patients (p<0.001, p<0.01 and p<0.001, respectively). Correlations between mRNA levels were significant for IL-12 and IL-4, IL-12 and IL-10, and IL-4 and IL-10 (p<0.05). The mRNA levels of IL-4 and IL-10 were very low compared to those of IL-12 in seven of the 30 patients with RA; all of these patients were in stage 4, and serum levels of CRP, ESR and blood platelet count which are considered as indices of the severity of inflammation, were significantly elevated in these seven patients compared to the other 23 RA patients. The markedly reduced synthesis of both IL-4 and IL-10 mRNA could be considered to be related to the progression and/or activity of RA. The results of this study therefore indicate an imbalance in the levels of Th1 and Th2 cytokines at the site of inflammation in RA, and draw attention to the possibility of treatment of progressive or intractable RA with IL-4 and/or IL-10.