Objective. To determine the relationship between timing and magnitude of Disease Activity Score [DAS28(ESR)] nonresponse (DAS28 improvement thresholds not reached) during the first 12 weeks of treatment with certolizumab pegol (CZP) plus methotrexate, and the likelihood of achieving low disease activity (LDA) at 1 year in patients with rheumatoid arthritis. Methods. In a post-hoc analysis of the RAPID 1 study, patients achieving LDA [DAS28(ESR) ≤ 3.2] at Year 1 were assessed according to DAS28 nonresponse at various timepoints within the first 12 weeks. Results. Seven-hundred eighty-three patients were included (CZP 200 mg, n = 393; CZP 400 mg, n = 390). A total of 86.9% of patients in the CZP 200 mg group had a DAS28 improvement of ≥ 1.2 by Week 12. Of the 13.1% of patients with DAS28 improvement < 1.2 by Week 12, only 2.0% had LDA at Year 1. Failure to achieve LDA at Year 1 depended on timing of nonresponse - 22.3%, 8.4%, and 2.0% of patients with DAS28 improvement < 1.2 by Weeks 1, 6, and 12, respectively, had LDA at Year 1 - and magnitude of initial lack of DAS28 improvement; for example, compared with the patients with DAS28 < 1.2 improvement, fewer patients with DAS28 < 0.6 had LDA at Year 1 (17.4%, 2.4%, and 0.0% at Weeks 1, 6, and 12, respectively). Conclusion. Failure to achieve improvement in DAS28 within the first 12 weeks of therapy was predictive of a low probability of achieving LDA at Year 1. Moreover, the accuracy of the prediction was found to be strongly dependent on the magnitude and timing of the lack of the response. The Journal of Rheumatology Copyright © 2012. All rights reserved.