Estimates of renal function are widely used in clinical practice and research. We assessed the performance of the Cockcroft-Gault (CG) and the Modification of Diet in Renal Disease (MDRD) equations in lupus nephritis patients. Data from ninety-seven lupus nephritis patients in the Hopkins Lupus Cohort were reviewed. Two renal function estimates, the CG and the MDRD, were compared with the 24 h creatinine clearance (CrCl). In the entire group of patients, the CG and MDRD equations had good global agreement with CrCl (R-square = 0.91 and 0.69, respectively). On average the CG equation overestimated CrCl by 2.36 mL/min/1.73 m(2), whereas the MDRD equation underestimated CrCl by 5.85 mL/min/1.73 m(2), P = 0.0004. The CG equation had greater accuracy (mean squared error) than the MDRD equation (14.93 versus 28.47 mL/min/1.73 m(2), P = 0.002) when predicting CrCl. Although both equations lacked precision (standard deviation of the difference scores) in patients with CrCl > or = 60 mL/min/1.73 m(2), the CG equation was more precise than the MDRD equation in this group, (15.68 versus 29.58 mL/min/1.73 m(2), P = 0.003). In lupus nephritis patients, the CG equation was superior to the MDRD equation as an estimate of CrCl. However, both equations lacked precision in patients with CrCl > or = 60 mL/min/1.73 m(2).