Kidney transplantation is the preferred treatment for permanent kidney failure. Following transplantation, the kidney function must be followed closely to detect problems so that investigations and appropriate treatment can be started early. Currently, function is monitored with the use of serum creatinine. In clinical trials involving kidney transplant recipients, markers of kidney function, such as serum creatinine, are increasingly being used as outcomes to evaluate new treatments. However, serum creatinine is not very accurate or sensitive at detecting change in kidney transplant function. Newer methods of evaluating kidney function, such as the Modification of Diet in Renal Disease (MDRD) equation and cystatin C, are known to be accurate markers of function in patients with non-transplant kidney disease. This study will compare the MDRD estimate and cystatin C estimate of kidney function with an accepted method (radioisotope clearance study) of measuring true kidney function in 250 renal transplant patients. Each patient will have 2 measurements made at least 3 months apart to determine the accuracy and responsiveness to change over time for the MDRD equation and cystatin C. If the results demonstrate that these new methods are accurate then clinical care and research studies involving transplant patients will be greatly enhanced. Patients and physicians would have a simple test that could detect problems earlier and more precisely monitor response to treatment leading to improved outcomes for renal transplant recipients.