Two studies are reported of patients with rheumatoid arthritis. The first was a retrospective-prospective-prospective study and comprised a cohort of 489 patients with rheumatoid arthritis followed for a mean of 12.2 years. Lymphoproliferative malignancies developed in 10 patients (2.2 percent) after a mean interval of 11.8 years. The second was a study of 30 patients, from various centers in England, with rheumatoid arthritis and lymphoproliferative malignancies. The effects of chronicity of rheumatoid arthritis, drug therapy, and possible predisposing factors in the etiology of the lymphoproliferative malignancies were examined. Cytotoxic drugs could not be implicated in the pathogenesis of the lymphoproliferative malignancies, but phenylbutazone and D-penicillamine may have played a role in some cases. Current evidence supports the hypothesis that chronic synovitis in rheumatoid arthritis is associated with persistent activation of lymphocytes in lymph nodes and eventual malignant transformation in some cases.