The National Kidney Foundation has advocated the use of the abbreviated Modification of Diet in Renal Disease (MDRD) equation to estimate glomerular filtration rate (GFR) from serum creatinine measurements in clinical laboratories. However, healthy persons were not included in the development of the MDRD equation. To assess the accuracy of the MDRD equation in patients with chronic kidney disease compared with healthy persons and to develop a new equation that uses both patients with chronic kidney disease and healthy persons. Cross-sectional study. The Mayo Clinic, a tertiary-care medical center. Consecutive patients (n = 320) who had an iothalamate clearance test specifically for chronic kidney disease evaluation and consecutive healthy persons (n = 580) who had an iothalamate clearance test specifically for kidney donor evaluation. Serum creatinine levels, GFR, demographic characteristics, and clinical characteristics were abstracted from the medical record. The MDRD equation underestimated GFR by 6.2% in patients with chronic kidney disease and by 29% in healthy persons. Re-estimated coefficients for serum creatinine and sex were similar to the original MDRD equation in the chronic kidney disease series but not in the healthy series. At the same serum creatinine level, age, and sex, GFR was on average 26% higher in healthy persons than in patients with chronic kidney disease (P < 0.001). A quadratic GFR equation was developed to estimate logarithmic GFR from the following covariates: 1/SCr, 1/SCr2, age, and sex (where SCr = serum creatinine). The new equation was not developed in a general population sample. Elderly and African-American persons were underrepresented. The MDRD equation systematically underestimates GFR in healthy persons. A new equation developed with patients who have chronic kidney disease and healthy persons may be a step toward accurately estimating GFR when the diagnosis of chronic kidney disease is unknown.