Variable treatment responses to glucocorticoids occur in patients with rheumatoid arthritis (RA). In renal transplantation and asthma treatment responses correlate with in vitro tests of glucocorticoid immunosuppression. We compared in vitro methylprednisolone suppression of concanavalin A stimulated cellular proliferation with clinical responses to methylprednisolone in patients with RA. Patients found to be glucocorticoid sensitive by in vitro testing had significantly greater improvements in joint score and soluble interleukin 2 receptor (sIL-2R) levels compared to control patients, indicating that individual responsiveness to glucocorticoid exists in RA. Similar in vitro and in vivo changes of sIL-2R levels suggests that they reflect cell mediated immune events in vivo.