Background Conventional estimates of glomerular dysfunction, including serum creatinine and creatinine clearance, are inadequate in older people In this study we have compared the diagnostic accuracy of a novel test of kidney disease, cystatin C, against these markers in older patients with a range of renal function. Methods Fifty-three patients (mean age 79.6 years, range 69-92 years) with a variety of medical diagnoses were recruited via outpatient clinics. Exclusion criteria included active rheumatoid disease, known current malignancy, renal replacement therapy/renal transplantation and cognitive impairment. Cr-51-EDTA was used as the reference method against which the other markers of glomerular filtration rate were compared using regression analyses. Results The best fit with glomerular filtration rate was given by Cockcroft and Gault calculated clearance (R-2=0.83), followed by serum cystatin C (R-2=0.79), serum creatinine (R-2=0.76) and creatinine clearance (R-2=0.73). The accuracy for glomerular filtration rate prediction was poor for all markers. Serum cystatin C detected nearly all patients with mild renal impairment whereas serum creatinine only detected half of these cases. Regression modelling predicted that the upper limit of normal for serum cystatin C would be exceeded as glomerular filtration rate fell below 64 mL/min/1.73 m(2), compared with 44 mL/min/1.73 m(2) for serum creatinine. Conclusion Serum cystatin C is a simple and sensitive screening test for kidney dysfunction in older people.