In rheumatoid synovium, interleukin-6 (IL-6) is the most abundantly expressed cytokine. Therefore, we investigated whether serum IL-6 levels could be clinically useful parameters of disease activity in rheumatoid arthritis (RA). The serum IL-6 levels were measured by enzyme-linked immunosorbent assay. The IL-6 levels were significantly higher in sera from patients with RA (n = 108, 13.3 +/- 14.0 pg/mL) than in those from the normal controls (n = 10, < 3.13 pg/mL). The IL-6 levels significantly correlated with conventional parameters, such as the Lansbury index, joint score, erythrocyte sedimentation rates, and C-reactive protein. In addition, the serum IL-6 level was useful to determine clinical remission. Advantages of serial measurement of IL-6 were observed in two cases. These findings suggested that the serum IL-6 level may be clinically useful as a sensitive parameter of disease activity in RA.