Platelet function tests were evaluated in 20 patients with rheumatoid arthritis (RA), 9 of whom displayed thrombocytosis (greater than 400,000 platelets/microliter). Most patients had an enhanced sensitivity to collagen and epinephrine induced aggregation, as compared with a reference group. In contrast, malondialdehyde (MDA) production, an index of platelet arachidonic acid metabolism, was in the normal range for all the patients except 3, who were LE phenomenon positive. The hypersensitivity of platelets to aggregating stimuli was particularly marked in thrombocytotic patients, who also showed the most striking biochemical and clinical abnormality. These findings indicate the crucial role of platelets in the development and self-sustaining of RA.