A simple method for estimating the rate of progression of chronic renal failure was evaluated in 11 dogs. The plot of the reciprocal of the serum creatinine concentration vs age declined linearly, and projection of the regression line to the abscissa accurately predicted each dog's age at the time of death attributable to renal failure. The linear decline of the reciprocal of the serum creatinine concentration vs age indicated that nephrons may be lost at a constant rate in dogs with chronic renal failure. Changes in the slope of this line may provide a means of evaluating response to treatment.