Interleukin-2 (IL-2) is an important growth factor for T lymphocytes. Its effects are mediated by cell surface receptors (IL-2 R) expressed on activated T cells. Receptor protein can be shed from cell membranes and the soluble form (sIL-2 R) is detectable by enzyme linked immunosorbent assay (ELISA). We have studied serial levels of sIL-2 R in the sera of patients with rheumatoid arthritis (RA). In 13 patients with active disease, the mean serum level of sIL-2 R was raised compared to age-matched healthy controls. In 48 samples taken at different times from 13 patients, serum sIL-2 R correlated significantly with Ritchie joint index, duration of early morning stiffness, patient pain score, physician's assessment, erythrocyte sedimentation rate (ESR) and platelet count. In individual patients, serial sIL-2 R serum levels fell with treatment preceding clinical improvement. In four patients where serum sIL-2 R levels fell and clinical improvement occurred, subsequent spontaneous increases of serum sIL-2 R level preceded increased clinical disease activity by up to 2 weeks. Serum sIL-2 R level in RA probably reflects activation of underlying immunopathogenic mechanisms and appears to be an excellent monitor of clinical disease activity. More importantly, a rising level may also predict exacerbation of disease activity.