These analyses assessed whether creatinine based estimates of glomerular filtration rate (eGFR) accurately represent (1) graft function at different times post-transplant and (2) changes in function over time. These analyses compared iothalamate GFR to eGFR in 684 kidney allograft recipients. Changes in graft function over time (GFR slope) were measured in 360 of 459 recipients (78%) who were followed for at least 3 years. Ninety-five percent of the patients were Caucasians and 72% received kidneys from living donors. All eGFR calculations correlated significantly with GFR at all time points. However, eGFR were less precise and less accurate during the first-year post-transplant than thereafter. The average rate of GFR change (slope) was −2.93 ± 11.3%/year (−1.06 ± 5.3 mL/min/1.73m2/year). Fifty-four percent of patients had stable or positive GFR slopes. The GFR and eGFR slopes were highly correlated. However, eGFR slope, particularly when calculated by MDRD, significantly underestimated the number of patients with declining graft function. For example, 165 out of 360 patients (46%) lost GFR faster than −1 mL/min/1.73m2/year. eMDRD identified only 83 of these patients (50%) while the eMayo formula identified 134 (81%). In conclusion, eGFR correlate with GFR but they have relatively low precision and accuracy particularly early post-transplant. eGFR slopes underestimate graft functional loss although some formulas are significantly better than others for this calculation.