Accurate estimation of the glomerular filtration rate (GFR) in patients with preeclampsia is often difficult or impossible to accomplish. In this study, the Cockcroft-Gault (CG), Modification of Diet in Renal Disease (MDRD), and MDRD2 formulas were evaluated for their accuracy in determining GFR in the setting of preeclampsia. The estimated GFR calculated from these formulas was compared with the creatinine clearance values obtained from a 24-hour urine collection in 209 preeclamptic patients recruited from five large hospitals. Additionally, a set of new equations that more accurately estimate GFR in preeclamptic patients based on ethnicity, preeclampsia GFR (PGFR), was created. Both the CG and MDRD formulas were inaccurate in predicting GFR in preeclamptic patients, and both were significantly less accurate than PGFR. In conclusion, current GFR estimation equations based on serum creatinine values in nonpregnant patients are not reliable measures of renal function in patients with preeclampsia. The use of a new (PGFR) formula is recommended.