Background: Patients with anorexia nervosa (AN) are at a high risk of renal failure. Chronic kidney disease (CKD) is often missed in these patients because the serum creatinine is a poor marker of kidney function. We studied the utility of cystatin C to detect renal failure in this population. Method: Twenty-seven AN patients were studied. Glomerular filtration rates (GFR) were measured with the chromium-51- ethylenediaminetetraacetate (51Cr-EDTA) method. We compared the ability of creatinine and cystatin C to detect stage 3 CKD (GFR below 60 ml/min) by ROC curve analysis. Results: In this cohort, there is no correlation between GFR and serum creatinine, but there is a significant correlation between cystatin C and GFR. By ROC analysis, the cystatin C concentration is better than the serum creatinine concentration for the detection of stage 3 CKD (area under the curve of 0.86 vs. 0.61, p = 0.05). Conclusion: Plasma cystatin C is better than serum creatinine in detecting stage 3 CKD in patients with AN.