Background: Certolizumab pegol is a pegylated humanized Fab' fragment that binds tumor necrosis factor α. Methods: In a randomized, double-blind, placebo-controlled trial, we evaluated the efficacy of certolizumab pegol in 662 adults with moderate-to-severe Crohn's disease. Patients were stratified according to baseline levels of C-reactive protein (CRP) and were randomly assigned to receive either 400 mg of certolizumab pegol or placebo subcutaneously at weeks 0, 2, and 4 and then every 4 weeks. Primary end points were the induction of a response at week 6 and a response at both weeks 6 and 26. Results: Among patients with a baseline CRP level of at least 10 mg per liter, 37% of patients in the certolizumab group had a response at week 6, as compared with 26% in the placebo group (P = 0.04). At both weeks 6 and 26, the corresponding values were 22% and 12%, respectively (P = 0.05). In the overall population, response rates at week 6 were 35% in the certolizumab group and 27% in the placebo group (P = 0.02); at both weeks 6 and 26, the response rates were 23% and 16%, respectively (P = 0.02). At weeks 6 and 26, the rates of remission in the two groups did not differ significantly (P = 0.17). Serious adverse events were reported in 10% of patients in the certolizumab group and 7% of those in the placebo group; serious infections were reported in 2% and less than 1%, respectively. In the certolizumab group, antibodies to the drug developed in 8% of patients, and antinuclear antibodies developed in 2%. Conclusions: In patients with moderate-to-severe Crohn's disease, induction and maintenance therapy with certolizumab pegol was associated with a modest improvement in response rates, as compared with placebo, but with no significant improvement in remission rates.