The purpose of this study is to investigate whether the drug BG00012 can improve the symptoms of rheumatoid arthritis (RA) in patients who are already taking methotrexate for RA, but who still suffer symptoms of RA. The purpose is also to ensure that the combination of methotrexate is safe for patients to take and that any side effects are acceptable to patients with RA.

The study is a double-blind, placebo-controlled study of BG00012 plus methotrexate, versus methotrexate alone, in patients with active RA, who have had an incomplete response to other conventional disease modifying anti-rheumatic (DMARD) therapy.

Inclusion Criteria:

• must be taking methotrexate
• inadequate response to at least one conventional DMARD
• swollen and tender joint count

Exclusion Criteria:

• previous treatment with TNF or any other biologic or prosorba column

Other criteria also apply.