Serum levels of soluble intercellular adhesion molecule-1 (sICAM-1), ceruloplasmin (Cp), and transferrin (Tf) were measured in patients with rheumatoid arthritis (RA) and the correlations of these parameters with disease activity were investigated. Serum sICAM- 1 levels were determined by a sandwich enzyme-linked immunosorbant assay (ELISA) in serums from 42 patients with RA and 30 healthy controls. Erythrocyte sedimentation rate (ESR) was determined by the Westergren method and C-reactive protein (CRP), Cp, and Tf by nephelometric methods. Disease activity was assessed by standard criteria. Serum Tf levels were significantly diminished and serum levels of sICAM-1 and Cp were significantly increased in patients with RA, compared to healthy controls. Serum sICAM-1 levels showed negative correlation with serum Tf levels (r = -0.47, p < 0.01), and positive correlation with serum Cp levels (r = 0.49, p < 0.001). There was weak positive correlation between sICAM-1 levels and the Ritche articular index (RAI) scores (r = 0.32, p <0.05) and serum CRP levels (r = 0.44, p <0.01), but no significant correlations of sICAM-1 levels with ESR, patient's age, or duration of disease. There were no significant correlations between values of serum CRP, RAI score, or ESR with serum CP or Tf levels. This study indicates that serum sICAM-1, together with other parameters, is a useful and novel marker for evaluating the disease status and activity of patients with RA.