We describe 2 patients with rheumatoid arthritis (RA) in whom non-Hodgkin's lymphomas developed during low dose pulsed methotrexate (MTX) treatment. The tumors regressed after discontinuation of MTX with no additional treatment. Serum levels of IgE increased concomitantly with the development of lymphoma, and decreased along with the regression of the lymphoma in both patients. These 2 cases and a review of the literature suggest that measuring serum IgE may have a predictive value for monitoring lymphoma in patients with RA treated with MTX.