The aim of the present study is to describe the influence of glucocorticoid immunosuppression on serum cystatin C concentration in renal transplant patients. To evaluate the influence of immunosuppressive regimens, espe- cially glucocorticoids, on serum cystatin C level, 38 clinically stable patients on immunosuppression therapy with low-dose glucocorticoids were compared to 30 clinically stable patients receiving cyclosporin A alone, and 18 clinically stable patients receiving cyclosporin A together with azathioprine. Clinical stability was defined as the absence of acute rejection, febrile infection, and cyclosporin A toxicity, as well as stability of creatinine clearance as estimated by the formula of Cockroft and Gault. All groups were compared for estimated creatinine clearance (CrCl) values and had comparable gender, age and time since transplantation. The group receiving short-course, high-dose methylprednisolone was analyzed at four time points: a) before methylprednisolone commencement (median, 15 days); b) the day methylprednisolone was introduced (before medication); c) after 3 days of methylprednisolone therapy; and d) on a follow-up 9-10 days after the last dose. Intravenous administration of high-dose methylprednisolone led to significant differences in cystatin C levels at different time points (before administration, after three doses, and 8 days after discontinuation). Glucocorticoid medication in adult renal transplant patients is associated in a dose-dependent manner with increased cystatin C, leading to systematic under estimation of GFR. Moreover, our data illustrate the need for specific reference intervals in patients on glucocorticoid therapy. In clinical routine settings, as well as in future clinical studies, it is important to take glucocorticoid medication into account when interpreting serum cystatin C concentrations in renal transplant patients presumably, as well as in other patient groups.