This single arm study will measure the effect of MabThera in combination with methotrexate on the progression of synovitis, the extent of bone marrow edema, and the number of erosions in the wrist and hand of patients with rheumatoid arthritis, using a new MRI technique. Patients will receive MabThera 1000mg i.v. on days 1 and 15, in combination with a stable dosage of 10-25mg/week methotrexate throughout the duration of the study. Further courses of MabThera will be provided to eligible patients. MRI will be performed 1-2 weeks before treatment initiation, and repeated 1 and 6 months after the first MabThera infusion. The anticipated time on study treatment is 1-2 years, and the target sample size is <100 individuals.