Advances in the conservative management of patients with chronic renal insufficiency have reaffirmed the need for the accurate measurement of glomerular filtration rate (GFR). The two most readily available indices of GFR are the urea clearance and the creatinine clearance; yet neither accurately reflects GFR in advanced renal disease. Theoretical considerations suggest thatwhen GFR is less than 20ml/minthe combined use of urea clearance and creatinine clearance should accurately measure GFR. This thesis was tested in patients with advanced renal insufficiency. In all instances, the mean value for the urea plus creatinine clearance closely approximated the simultaneously determined inulin clearance. Since analyses for both urea and creatinine may be performed on the same samples of blood and urine, the present technique offers an accurate clinical means of measuring GFR in advanced chronic renal disease.