Background: Certolizumab pegol is a pegylated humanized Fab' fragment with a high binding affinity for tumor necrosis factor (alpha) that does not induce apoptosis of T cells or monocytes.Methods: In our randomized, double-blind, placebo-controlled trial, we evaluated the efficacy of certolizumab pegol maintenance therapy in adults with moderate-to-severe Crohn's disease. As induction therapy, 400 mg of certolizumab pegol was administered subcutaneously at weeks 0, 2, and 4. Patients with a clinical response (defined as reduction of at least 100 from the baseline score on the Crohn's Disease Activity Index [CDAI]) at week 6 were stratified according to their baseline C-reactive protein level and were randomly assigned to receive 400 mg of certolizumab pegol or placebo every 4 weeks through week 24, with follow-up through week 26.Results: Among patients with a response to induction therapy at week 6 (428 of 668 [64%]), the response was maintained through week 26 in 62% of patients with a baseline C-reactive protein level of at least 10 mg per liter (the primary end point) who were receiving certolizumab pegol (vs. 34% of those receiving placebo, P<0.001) and in 63% of patients in the intention-to-treat population who were receiving certolizumab pegol (vs. 36% receiving placebo, P<0.001). Among patients with a response to induction therapy at week 6, remission (defined by a CDAI score of lessthan/equal 150) at week 26 was achieved in 48% of patients in the certolizumab group and 29% of those in the placebo group (P<0.001). The efficacy of certolizumab pegol was also shown in patients taking and those not taking glucocorticoids or immunosuppressants and in patients who had and those who had not previously taken infliximab. Infectious serious adverse events (including one case of pulmonary tuberculosis) occurred in 3% of patients receiving certolizumab pegol and in less than 1% of patients receiving placebo. Antinuclear antibodies developed in 8% of the patients in the certolizumab group; antibodies against certolizumab pegol developed in 9% of all patients who entered the induction phase.Conclusions: Patients with moderate-to-severe Crohn's disease who had a response to induction therapy with 400 mg of certolizumab pegol were more likely to have a maintained response and a remission at 26 weeks with continued certolizumab pegol treatment than with a switch to placebo.