This study (RA0078) will continue to assess the safety, tolerability, and efficacy of Certolizumab Pegol (CZP) for 6 months as additional medication to methotrexate (MTX) with or without folic acid in Chinese subjects with active Rheumatoid Arthritis (RA) who participated in the main feeder study, RA0044. All subjects will continue to receive their established treatment with MTX with or without folic acid. The dose of MTX may be decreased by the Investigator due to toxicity, but should not be discontinued completely. Concomitant nonsteroidal anti-inflammatory drugs and oral corticosteroids will be permitted. For each subject, the study duration will last a maximum of approximately 32 weeks.