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The glomerular filtration rate (GFR) is the main indicator of kidney function. In clinical practice the GFR is often estimated from serum creatinine. In the elderly, serum creatinine is notoriously unreliable as an estimator of GFR. Recently, serum cystatin C has been proposed as a new endogenous marker of glomerular filtration rate. A total of 144 patients, aged more than 60 years (mean age 70.4 years), who had undergone 51CrEDTA clearance, were enrolled in our study. In each patient serum creatinine and serum cystatin C were determined. The reciprocal of serum creatinine, the reciprocal of serum cystatin C and creatinine clearance (from Cockcroft and Gault formula) were calculated. Serum cystatin C was measured with the particle-enhanced immunonephelometric method. The mean 51CrEDTA clearance was 34.5+/-25.55 ml/min/1.73 m2, the mean serum creatinine was 312+/-210 micromol/l and the mean serum cystatin C 3.15 mg/l+/-1.62 mg/l. We found a significant correlation between 51CrEDTA clearance and serum creatinine, serum cystatin C, the reciprocal of serum creatinine and the reciprocal of serum cystatin C as well as with creatinine clearance. In comparison of the correlation coefficients we found that the correlation between 51CrEDTA clearance and serum cystatin C was significantly better than that with serum creatinine (p0.05). The correlation between 51CrEDTA clearance and the reciprocal of serum cystatin C was superior to that with the reciprocal of serum creatinine (p0.003) and calculated creatinine clearance (p0.003). Our results indicate that serum cystatin C is a more reliable marker of GFR in the elderly than serum creatinine or creatinine clearance.