An estimating equation is derived with the use of regression techniquesto model the observed relation between the serum level of the marker and themeasured GFR in a study population. The Cockcroft and Gault formuladeveloped in 1973 and the MDRD developed in 1999 are the most commonformulae used in estimating GFR.The aim of this study was to investigate the most appropriate formulawhether MDRDb, MDRDw or CG to use in estimating glomerular filtrationrate in adult Egyptian males and females.This study was conducted on eighty persons, forty of whom wereknown as CKD attending the Nephrology Department, Theodor BilharzResearch Institute (TBRI), from October 2007 till December 2007, and fortyapparently healthy individuals.We determined the level of Cystatin C in serum for all cases together withhistory taking, clinical examination, kidney function tests, routineinvestigations and conventional ultrasound.We concluded that in the healthy, adult, Egyptian, males and females themost appropriate formula for estimation of GFR is the MDRDw, as it showedthe nearest mean value to Cystatin C.in adult Egyptian males and females with CKD the most appropriate formulafor estimation of GFR is the MDRDb, as it showed the nearest mean value toCystatin C. The difference between Cystatin clearance and Creatinineclearance is approximately 1ml/min/1.73m2, so Creatinine remains a goldenstandard for measuring GFR in Egypt as it is easy, cheap and a relativelyaccurate measure for estimation of GFR.