In 31 of 34 patients with chronic renal insufficiency caused by various diseaes, reciprocal serum-creatinine concentration declined linearly as creatinine concentration rose from a mean of 2-6 mg/dl to 14-8 mg/dl over an average of 71 months. These results indicate that in most cases reciprocal serum-creatinine declines linearly with time as chronic renal failure progresses. Analysis of this relation in individual patients gives an estimate of the progression of the disease, may help to determine the effects of therapy, and could be used to predict when dialysis will become necessary.