Serum creatinine remains the most widely used measure of renal function in clinical practice and in clinical trials. However, the serum concentration reflects not only renal excretion, which is composed of filtration and tubular secretion, but also the generation, intake, and metabolism of creatinine. Thus, serum creatinine does not provide an adequate estimate of glomerular filtration rate (GFR). Creatinine clearance is also an inaccurate and imprecise measure of GFR in clinical practice. The questions concerning the abnormality or the change of glomerular function can be answered with greater accuracy and precision by serum creatinine if the factors affecting its concentration are taken into account. The slope of the decline in reciprocal serum creatinine versus time does not accurately reflect changes in creatinine clearance and does not allow an accurate assessment of the rate of progression of renal disease. It can even be hazardous to use such value to observe the efficacy of treatments for progressive renal diseases.