In 85 patients with established chronic renal failure, as manifested by serum creatinine concentration ([Cr]) > 0.177 mM (> 2 mg/dl), urinary clearance of 99mTc-DTPA (GFR) and [Cr] were determined one to four times and averaged. GFR was expressed per 3 m2 of height2 rather than in terms of 1.73 m2 of surface area because weight changes but height does not. GFR in these patients averaged 13 ml/min/3 m2 (range 2 to 37). The following equations were derived for predicting GFR from [Cr]-1 (mM-1), age (years) and weight (kg): males, GFR = 7.57 [Cr]-1 - 0.103 age + 0.096 weight - 6.66; females, GFR = 6.05 [Cr]-1 - 0.08 age + 0.08 weight - 4.81. These equations predict GFR in this series with a root mean square error of 3.0 ml/min and account for 89% of the observed variation in GFR. They are applicable only to patients with [Cr] > 0.177 mM (2 mg/dl). More precise estimates can be obtained by including a value for 24-hour urinary urea N, as a measure of protein intake. Less precise but simpler estimates can be obtained from [Cr]-1 alone: males, GFR = 7.58 [Cr]-1 - 4.29; females, GFR = 6.11 [Cr]-1 - 3.8. Expressing [Cr] in mg/dl, the latter equations become: males, GFR = 86 [Cr]-1 - 4.29; females, GFR = 69 [Cr]-1 - 3.8.