To date, little evidence is available to define the role of cystatin C in patients with renal transplants. Thus, to assess, whether cystatin C (CysC) provides better information on renal function than other markers, CysC, creatinine clearance (CrCl), serum creatinine (SCr), beta2-microglobulin (beta2-M), and 125I-Iothalamate clearance were determined in 30 patients. Correlation and ROC curves were obtained and characteristics like sensitivity and specificity were calculated. Further, to evaluate the usefulness of these markers for monitoring, intraindividual coefficients of variation for CysC and SCr measurements were compared in 85 renal transplant patients. CysC correlated best with GFR, whereas SCr, CrCl and beta2-M all had lower correlation coefficients. CysC was superior to SCr, even when renal function equations of were used. The diagnostic accuracy of CysC was significantly better than SCr. but did not differ significantly from CrCl and beta2-M. Together, our data show that in patients with renal transplants, CysC has a similar diagnostic value as CrCl. However, it is superior to determinations of SCr. The intraindividual variation of CysC is significantly greater than that of SCr. This might be due to better ability of CysC to reflect temporary changes especially in mildly impaired GFR, most critical for early detection of rejection and other function impairment. In conclusion, CysC allows for easy and accurate assessment of renal function (GFR) in steady state renal transplant patients and is clearly superior to the commonly used serum creatinine.