To determine the effect of tocilizumab (TCZ) on reactive oxygen species (ROS) in 11 patients with rheumatoid arthritis (RA), reactive oxygen metabolites (d-ROM) were measured using a Free Radical Elective Evaluator. The Disease Activity Score (DAS28) and matrix metalloproteinase-3 (MMP-3) level were also evaluated. d-ROM measured 392 ± 110 Carratelli units [U. CARR] on initiation of TCZ, and significantly decreased to 237 ± 82, 248 ± 88, and 226 ± 91 U.CARR after 3, 6, and 12 months, respectively (p < 0.05). The DAS28-4-ESR was 4.77 ± 1.0 on initiation of TCZ, and significantly decreased to 2.19 ± 1.23, 1.51 ± 0.71, and 1.48 ± 0.48 after 3, 6, and 12 months, respectively (p < 0.05). Serum MMP-3 level was 197 ± 150 ng/ml on initiation of TCZ, and was significantly decreased to 92 ± 56, 73 ± 53, and 69 ± 41 ng/ml after 3, 6, and 12 months, respectively (p < 0.05). Pearson analysis showed that d-ROM value was significantly correlated with DAS28 (r = 0.543, p < 0.05), but not with MMP-3 (r = 0.174, p = 0.29). TCZ was found to rapidly and significantly decrease ROS in patients with RA, and d-ROM value may be useful as a marker of disease activity of RA.