The work was devised to compare measurements of glomerular filtration rate (GFR) by technetium-99m-diethylenetriaminepentacetic acid (<sup>99m</sup>Tc-DTPA) renogram to those by creatinine clearance (measured and predicted by Cockroft and Gault) and by inulin clearance. A total number of 65 individuals were enrolled: 15 healthy controls and 50 patients with renal disease. Compared to inulin clearance used as the gold standard, <sup>99m</sup>Tc-DTPA overestimated at low and underestimated at high GFRs. <sup>99m</sup>Tc-DTPA measurements were less precise than creatinine clearance except for individuals with GFR >100 ml/min × 1.73 m<sup>2</sup>. Measured creatinine clearance had the highest correlation coefficient with inulin clearance, <sup>99m</sup>Tc-DTPA clearance the lowest. In correlation analyses, 81.5% of the interindividual variability for measured creatinine clearance could be explained by true differences in inulin clearance; this value dropped to 59.1 and 57.4% for predicted creatinine clearance and <sup>99m</sup>Tc-DTPA, respectively. In patients with GFR <25 ml/min × 1.73 m<sup>2</sup>, all <sup>99m</sup>Tc-DTPA measurements were out of the 95% confidence interval for the inulin measurement. It can be inferred that <sup>99m</sup>Tc-DTPA clearance from the renogram is less precise than measured and predicted creatinine clearance.