BACKGROUND: It has been suggested that the glomerular filtration rate can be predicted on the basis of serum cystatin C concentration. The aim of this study was to investigate the clinical utility of serum cystatin C as a marker of renal function and to evaluate the effect of an initial dose of vancomycin using serum cystatin C concentration in elderly patients. MATERIALS AND METHODS: Data was collected from July 2006 to September 2008. The subjects were elderly patients (> or =60-years-of-age; n=number of patients) who had been treated with vancomycin. Serum concentrations of parameters including vancomycin, creatinine, and cystatin C were measured. Correlations between measured vancomycin concentrations and predicted vancomycin concentrations based on serum cystatin C or serum creatinine were compared, and the initial dose setting of vancomycin on the basis of serum cystatin C concentration was evaluated. RESULTS: Serum cystatin C was a good marker of renal function in comparison with serum creatinine for various doses of vancomycin in the elderly subject. In subpopulation (n=88) with normal serum cystatin C level (< or =1.2 mg/L) who were treated with routine adult vancomycin dose (1g q 12 hours), vancomycin concentration was within the therapeutic range (5-15 mg/L) in 68 patients (77.3%), and was <20 mg/L in 77 of the 88 patients (88.5 %). CONCLUSIONS: Predictive prowess of serum vancomycin concentrations on the basis of the serum cystatin C concentration could be better than that based on the serum creatinine concentration in elderly patients.