In clinical practice, the measurement of endogenous serum substances in order to estimate glomerular filtration rate (GFR) is commonly performed, and the serum creatinine level has become the most commonly used serum marker of renal function. However, the measurement of the serum creatinine concentration can sometimes lead to an overestimation of GFR, especially in the elderly. In recent years, it has been suggested that GFR can be predicted based on the serum cystatin C concentrations and that the serum cystatin C concentration is not influenced by gender or age. A recent meta-analysis demonstrated that serum cystatin C is a better marker for GFR than serum creatinine. In clinical practice, it has been suggested that serum cystatin C can optimize early detection for diabetic or hypertensive nephropathy. In addition, the use of serum cystatin C is possibly more appropriate for establishing an appropriate dose adjustment of drugs that are mainly eliminated by the kidney.