Short-term outcomes in renal transplantation, such as the acute rejection rate, have improved dramatically over the past decade. Unfortunately, this success has made it more difficult to evaluate new therapies in kidney transplantation. Markers of kidney function, such as serum creatinine and creatinine clearance, are now being used to evaluate kidney transplant function. However, serum creatinine and creatinine clearance have many limitations and correlate poorly with the glomerular filtration rate (GFR). The Modification of Diet in Renal Disease (MDRD) formula has been shown to be very accurate at predicting GFR in patients with kidney disease who don't have renal transplants. Cystatin C, a novel marker of renal function, has also been shown to be accurate in transplant and non-transplant patients. However, the MDRD formula and cystatin C have not been properly validated in a large sample of renal transplant recipients.