The ability to estimate rather than measure the glomerular filtration rate (GFR) in patients before and after liver transplantation would be helpful in estimating risk, dosing drugs, and assessing long-term toxicity of calcineurin inhibitors. Currently available equations for estimating the GFR have not been validated in either the pre- or post-liver transplant population. We have evaluated the performance of currently used formulas for the estimation of the GFR in this setting. Data were collected prospectively on patients who underwent liver transplantation between 1984 and 2001. GFR per 1.73 m2 was measured by I125 iothalamate in patients at the pretransplant evaluation and at 3 months, 1 year, and yearly posttransplant thereafter. GFR estimated by the Cockcroft-Gault equation, the Nankivell equation, and the equations from the Modification of Diet in Renal Disease (MDRD) Study (6, 5, and 4 variables) was compared with the measured GFR. Pretransplant GFR was available in 1,447 patients. The mean GFR was 90.7 ± 40.5 mL/min. Values for r and r2 were highest for the MDRD Study 6-variable equation (0.70 and 0.49, respectively). Only 66% of estimates were within 30% of the measured GFR. At 3 months, 1 year, and 5 years posttransplant, the mean GFR was 59.5 ± 27.1 mL/min, 62.7 ± 27.8 mL/min, and 55.3 ± 26.1 mL/min, respectively. Values for r and r2 for the MDRD Study 6-variable equations at 1 and 5 years posttransplant were 0.74 (0.55) and 0.76 (0.58), respectively. At these time points, however, only 67% and 64% of the estimated GFR were within 30% of the measured GFR. MDRD Study equations had greater precision than other equations, but the precision was lower than reported for MDRD estimation of GFR in other populations. Better methods for estimating the GFR are required for evaluation of renal function before and after liver transplantation. (Liver Transpl 2004;10:301–309.)