The risk of nephrogenic systemic fibrosis after gadolinium exposure is inversely related to renal function. Various methods are available to assess the glomerular filtration rate (GFR). Prediction formulas based on serum creatinine, such as the abbreviated Modification of Diet in Renal Disease (MDRD) formula, are most commonly used and appear acceptable for clinical purposes in the majority of patients with chronic renal failure. However, especially in patients at the extremes of body composition, the results from creatinine-based equations should be interpreted with caution. In those patients, additional methods, such as timed urine collections, predictions based on cystatin, single-shot radiotracer methods, or, optimally, inulin clearance could be considered. In this review, the strengths and limitations of different methods to assess GFR are discussed. Apart from inulin clearance, no method can be considered the gold standard in the assessment of GFR. In cases of doubt, the decision to use gadolinium-enhanced magnetic resonance imaging should always be based on clinical risk-benefit judgment. J. Magn. Reson. Imaging 2009;30:1341-1346. (c) 2009 Wiley-Liss, Inc.