Rituximab is the first monoclonal antibody to have been registered for the treatment of B-cell lymphomas. Randomized studies have demonstrated its activity in follicular lymphoma, mantle cell lymphoma, and diffuse large B-cell lymphoma in untreated or relapsing patients. Rituximab has transformed the outcome of these patients because of its high activity and low toxicity. A combination of rituximab plus cyclophosphamide, doxorubicin, vincristine, and prednisone, has the highest efficacy ever described with any chemotherapy in diffuse large B-cell lymphoma and follicular lymphoma.