X-ray fluorescence analysis provides a simple method of measuring glomerular filtration rate from the clearance of ordinary unlabeled radiographic contrast media. To measure the accuracy of iohexol clearance using a clinically simple 2-sample single injection technique, a comparison was made in 30 patients with classical continuous infusion techniques for plasma (y = 1.03x - 0.015, r = 0.981) and renal (y = 0.950x + 7.26, r = 0.964) iohexol clearance. A comparison was made with classical inulin clearance. The renal clearance of iohexol and inulin was virtually identical (y = 0.998x + 2.31, r = 0.986). Single injection iohexol clearance agreed closely with the renal inulin clearance (y = 0.947x + 4.92, r = 0.983) as did clearance based on a single 3-hour sample (y = 0.875x + 12.63, r = 0.962). Iohexol clearance using x-ray fluorescence is a simple, rapid and accurate alternative to inulin clearance for clinical and research purposes.