In this study we evaluated the use of cimetidine on the performance of glomerular filtration rate (GFR) estimating equations and 24-hour creatinine clearance (24-hour CrCl) against radionuclide GFR in 43 heart transplant recipients with stable renal function. Pearson correlation coefficients for the 24-hour CrCl, Cockcroft-Gault, Nankivell and Modification of Diet in Renal Disease study (MDRD) equations without cimetidine were 0.76, 0.70, 0.81 and 0.76, and cimetidine-aided coefficients were 0.82, 0.80, 0.80 and 0.72, respectively. All the tested equations without cimetidine use predicted GFR with moderate accuracy in this population. The addition of cimetidine improved the predictive ability of the 24-hour CrCl and Cockcroft-Gault formulas, but not the Nankivell and MDRD GFR estimating equations, in heart transplant recipients.