Eight hundred studies of glomerular filtration rate (GFR) measurements were performed by the standard slope-intercept method using [99mTc]DTPA and results were compared with a simultaneous measurement of the 3-hr tracer volume of distribution. A wide range of human renal function was studied and a nonlinear relationship between GFR and the volume of distribution resulted with an excellent correlation (r = 0.989). Agreement between the two measured parameters was not constant for all levels of renal function with the greatest accuracy being found for GFR = 60 to 100 ml/min.