Background/Aims: Despite widespread interest in determining the glomerular filtration rate (GFR) of patients, current methods all have significant limitations. Therefore, a compelling need exists for new tests of GFR that are both accurate and easy to perform. We have previously reported that the technique of neutron activation (NA) accurately measures iohexol in vitro. In this study, we demonstrate that NA can be used to determine GFR by measuring the clearance of iohexol, and directly compare these results to a gold-standard method based on <sup>99m</sup>Tc-DTPA. Methods: We studied 57 patients with mild to moderate chronic kidney disease and normal volunteers. Subjects were simultaneously injected with iohexol and <sup>99m</sup>Tc-DTPA. Blood and urine samples were collected over 4 h to calculate GFR by the UV/P method. Results: The range of GFRs was 28–212 ml/min. GFRs obtained using iohexol and <sup>99m</sup>Tc-DTPA correlated closely (R = 0.95). The bias between the 2 techniques was 0.96 ml/min, and precision (defined as the standard deviation of the mean of the difference between the 2 values for each patient) was 10.6 ml/min. Accuracy was such that 98% of subjects had NA GFRs within 20% of the reference <sup>99m</sup>Tc-DTPA measurements. Conclusions: We conclude that NA is an excellent technique to measure GFR. NA has several advantages over current methods to directly measure GFR, including the ability to reassay samples, high throughput and the avoidance of patient and hospital radioactivity exposure. In the future, NA could be applied to GFR agents that do not contain iodine, such as Gd-DTPA, and to the simultaneous measurement of agents that reflect renal blood flow, such as iodohippurate. Therefore, NA holds great potential to improve the measurement of renal function in a safe, easily obtainable way.