We aimed to demonstrate the incidence of serious respiratory infections in patients with rheumatoid arthritis (RA) treated with tocilizumab (TCZ) monotherapy. We analyzed the incidence of serious respiratory infections in 601 RA patients enrolled in TCZ clinical trials and their extension studies (TCZ cohort) and in 601 age- and sex-standardized RA patients treated in daily clinical practice at Tokyo Women's Medical University (IORRA subsample cohort). The rates of serious respiratory infections were 1.77 per 100 patient-years from 1999 to 2008 in the TCZ cohort and 0.53 per 100 patient-years from 2000 to 2009 in the IORRA subsample cohort. With the IORRA subsample cohort regarded as a standard population, the standardized incidence ratio (SIR) of serious respiratory infection in the TCZ cohort was 3.64 [95% confidence interval (CI) 2.56-5.01], standardized for age and sex; 2.35 (95% CI 1.66-3.24), standardized for age sex, and corticosteroid use; 1.85 (95% CI 1.30-2.55), standardized for age sex, and pre-existing pulmonary involvement; and 2.41 (95% CI 1.68-3.34) standardized for age sex, and disease activity. The risk of serious respiratory infection in the TCZ cohort was approximately double that in the IORRA subsample cohort after standardizing for corticosteroid use, pre-existing pulmonary involvement, or disease activity. This is comparable to the risk reported when tumor necrosis factor (TNF) inhibitors are used.