This randomized, double-blind, placebo controlled, parallel group study will assess the efficacy and safety of rituximab [MabThera/Rituxan] in patients with severe active rheumatoid arthritis refractory to at least one anti-TNF treatment, using magnetic resonance imaging (MRI) of the hand for efficacy measurement. Patients will be randomized to receive 2 intravenous infusions 14 days apart of either 1000mg MabThera (group A) or placebo (group B). All patients will receive methotrexate 10-25mg weekly. Patients in group A demonstrating clinical response at week 24 may receive further cycles of MabThera, patients in group B without clinical response will also be treated with active MabThera. Anticipated time on study treatment is up to 2 years. Target sample size is <50 patients.