Abstract Aims/hypothesis: We compared the predictive performance of a GFR based on serum cystatin C levels with commonly used creatinine-based methods in subjects with diabetes. Subjects, materials and methods: In a crosssectional study of 251 consecutive clinic patients, the mean reference (plasma clearance of 99m Tc-diethylene-triaminepenta-acetic acid) GFR (iGFR) was 88±2 ml min . A regression equation describing the relationship between iGFR and 1/cystatin C levels was derived from a test population (n=125) to allow for the estimation of GFR by cystatin C (eGFR-cystatin C). The predictive performance of eGFR-cystatin C, the Modification of Diet in Renal Disease 4 variable formula (MDRD-4) and Cockcroft-Gault (C-G) formulas were then compared in a validation population (n=126). Results: There was no difference in renal function (ml min −1 1.73 m −