Beside lymphocytes and neutrophils, eosinophils are also involved in the inflammatory reaction in rheumatoid arthritis (RA). In this study, adhesion characteristics of peripheral blood eosinophils were studied in 43 RA patients and 19 controls, together with the expression of the β2-integrin Mac-1 (CD11b/CD18). In addition, the production of oxygen radicals of isolated peripheral blood eosinophils and serum levels of eosinophil cationic protein (ECP) were measured in order to evaluate eosinophil activation. Adhesion of eosinophils to unstimulated human vascular endothelium was significantly higher in RA patients with active disease (n= 4) compared with controls (n= 14) (P < 0.005) and compared with patients with less active RA (n= 16) (P < 0.05). Nevertheless, the expression of the adhesion molecule Mac-1 (CD11b/CD18) was not increased in RA patients. ECP levels were higher in RA patients with active disease (P < 0.01). Release of oxygen radicals in response to phorbol stimulation was significantly elevated in active RA compared with controls (P < 0.05) and to less active RA (P < 0.05). We conclude that eosinophils of RA patients, especially those with active disease, are activated or at least primed and are involved in the inflammatory process in RA, analogous to the inflammation in asthma. The higher adhesion to inflamed endothelium is indicative of a higher infiltration in the joints, where tissue damage can be caused by toxic oxygen radicals and by granular proteins, such as ECP.