Background: Disease activity of rheumatoid arthritis (RA) is evaluated by composite measures, such as Disease Activity Score (DAS). Recently, much attention has been paid to a neutrophil-lymphocyte (N/L) ratio to evaluate the prognosis and the efficacy of intervention in various diseases. To determine whether the N/L ratio is a prognostic marker or a surrogate marker of response to biologics, this study investigated the N/L ratio in RA patients treated with biological agents. Methods: The medical records were reviewed of 358 patients with RA in routine care who were treated with infliximab (144 patients), etanercept (120 patients), adalimumab (25 patients), tocilizumab (41 patients), or abatacept (28 patients). The 28-joint DAS (DAS28), a hemogram, erythrocyte sedimentation rate (ESR), and serum levels of C-reactive protein and matrix metalloproteinase 3 were assessed at baseline and 6 months after the treatment. Results: The average N/L ratio significantly decreased from 5.9 at baseline to 4.5 6 months after the treatment. The N/L ratio and the DAS28-ESR, both at baseline and 6 months after the treatment, were modestly but significantly correlated. The N/L ratio was greater in patients with high disease activity than in patients with low disease sactivity. The change of the N/L ratio (ΔN/L) and the change of the DAS28-ESR were modestly but significantly correlated. Regarding the therapeutic response, the N/L ratio at baseline showed no significant difference between the response criteria; however, the N/L ratio after 6 months of treatment and the ΔN/L ratio differed significantly. The ΔN/L was also significantly correlated with the change of the serum level of C-reactive protein and the change of the DAS28-ESR. Conclusion: The N/L ratio is a marker of disease activity in RA. The ΔN/L ratio reflects the efficacy of biological agents but does not predict the response to biological agents.