Clinical assessment of kidney function is part of routine medical practice for adults, essential for assessing overall health, interpreting signs and symptoms, dosing drugs that are excreted by the kidneys, preparing for invasive diagnostic or therapeutic procedures, and detecting, evaluating and monitoring acute and chronic kidney diseases. The glomerular filtration rate (GFR) is considered the best overall index of kidney function in health and disease. GFR cannot be measured easily in clinical practice. Instead, GFR is estimated from equations using serum creatinine, age, race, sex and body size (1,2). One such equation, the Modification of Diet in Renal Disease (MDRD) Study equation, has gained widespread acceptance (3,4), and estimated GFR using this equation is reported by most clinical laboratories when measurement of serum creatinine is ordered (5). The MDRD Study equation is also used to assess the burden of chronic kidney disease in epidemiologic studies and public health (6). Prevalence of CKD in the U.S. has increased from approximately 10% in 1988-1994 to 13% in 1999-2004, corresponding to approximately 26.3 million people in 2000 (6,7).The MDRD Study equation was developed in people with CKD, and as such its major limitations are imprecision and systematic underestimation of measured GFR (bias) at higher levels (8). Our objectives were to develop and validate a new estimating equation based on serum creatinine that would be as accurate as the MDRD Study equation at GFR less than 60 ml/min/1.73 m2 and more accurate at higher GFR. We report development and validation of a new equation and compare it to the MDRD Study equation for estimating measured GFR and US prevalence of chronic kidney disease.