Accurate measurement of glomerular filtration rate (GFR) is complicated and costly; therefore, GFR is commonly estimated by assessing creatinine or cystatin C concentrations. Because estimates based on cystatin C predict cardiovascular disease better than creatinine, these estimates have been hypothesized to be superior to those based on creatinine, when the GFR is near the normal range. To test this, we measured GFR by iohexol clearance in a representative sample of middle-aged (50-62 years) individuals in the general population, excluding those with coronary heart or kidney disease, stroke or diabetes mellitus. Bias, precision (median and interquartile range of estimated minus measured GFR (mGFR)), and accuracy (percentage of estimates within 30% of mGFR) of published cystatin C and creatinine-based GFR equations were compared in a total of 1621 patients. The cystatin C-based equation with the highest accuracy (94%) had a bias of 3.5 and precision of 18 ml/min per 1.73 m², whereas the most accurate (95%) creatinine-based equation had a bias of 2.9 and precision of 15 ml/min per 1.73 m² The best equation, based on both cystatin C and creatinine, had a bias of 7.6 ml/min per 1.73 m², precision of 15 ml/min per 1.73 m², and accuracy of 92%. Thus, estimates of GFR based on cystatin C were not superior to those based on creatinine in the general population. Hence, the better prediction of cardiovascular disease by cystatin C than creatinine measurements, found by others, may be due to factors other than GFR.