• Thirty six patients suffering from active rheumatoid arthritis despite continued treatment with methotrexate will be evaluated for improvement of disease activity when taking GLPG0634 or matching placebo for 4 weeks.
• During the course of the study, patients will also be examined for any side effects that may occur, and the amount of GLPG0634 present in the blood as well as the effects of GLPG0634 on disease- and mechanism of action-related parameters in the blood will be determined.

Inclusion Criteria:

• Have active RA as shown by five or more swollen joints (from the 66-joint count), five or more tender joints (from 68-joint count), and a serum CRP ≥1.0 mg/dL;
• Have received methotrexate for six months or longer and at a stable dose of 7.5 to 25 mg/week (extremes included) for at least four weeks prior to screening and willing to continue on this regimen for the duration of the study;
• If taking oral steroids, these should be at a dose ≤10 mg/day of prednisone or prednisone equivalent and stable for at least four weeks prior to screening;
• If taking non-steroidal anti-inflammatory drugs (NSAIDs), these must be at a stable dose for at least two weeks prior to screening;
• Female subjects must have a negative pregnancy test unless they are surgically sterile or have been post-menopausal for at least one year (12 consecutive months without menses);
• Women of childbearing potential must use a medically acceptable means of birth control and agree to continue its use during the study and for at least four weeks after the last dose of study drug. Sexually active men must agree to use a medically acceptable form of contraception during the study and continue its use for at least 3 months after the last dose of study drug; and
• Able and willing to sign the informed consent prior to screening evaluations and agree to schedule of assessments.

Exclusion Criteria:

• Treatment with disease-modifying antirheumatic drugs (DMARDs), other than background methotrexate;
• Current or previous RA treatment with a biological agent, with the exception of biologics administered in a clinical study setting more than six months prior to screening (12 months for rituximab or other B cell depleting agents);
• Previous treatment at any time with a cytotoxic agent, other than methotrexate, before screening;
• Receipt of an intra-articular or parenteral corticosteroid injection within four weeks prior to screening;
• Known hypersensitivity to study drug ingredients or a significant allergic reaction to any drug as determined by the Investigator, such as anaphylaxis, requiring hospitalization;
• Positive serology for human immunodeficiency virus (HIV)1 or 2 or hepatitis B or C, or any history of hepatitis from any cause with the exception of hepatitis A;
• History of any inflammatory rheumatological disorders other than RA;
• History of tuberculosis (TB) infection;
• Pregnant or lactating women.