t In 1976, Cockcroft and Gault introduced a widely used formula comprising several anthropometric variables to compensate for the inadequacies of creatinine level as a marker of glomerular filtration rate (GFR). The present work investigates the possibility of introducing cystatin C-based formulas without anthropometric variables to predict GFR, determined by an invasive "gold standard" procedure (iohexol clearance), and to compare the diagnostic efficiency of such formulas with that of Cockcroft and Gault. All 451 adult patients referred to the University Hospital for determination of GFR by iohexol clearance measurements during a period of 6 months were included in the study. Calculations of bias (median percent error), correlation (adjusted R2), and accuracy (percentage of estimates within 30 and 50% of iohexol clearance) were used in the comparison. The cystatin C-based formula GFR (ml/min)=89.12 x cystatin C(-1.675) had lower bias and higher accuracy in predicting GFR than the Cockcroft-Gault formula. If a cystatin C-based formula including gender was constructed: GFR (ml/min)=99.19 x cystatin C(-1.713) x (0.823 for women), an even lower bias and higher accuracy were obtained. It is suggested that measurement of cystatin C should be used for the initial prediction of GFR of a patient.