Recent studies indicate that serum cystatin C is a better marker of glomerular filtration rate (GFR) and is a stronger predictor of cardiovascular disease and mortality than serum creatinine. Before cystatin C can gain wide acceptance, information about factors that affect generation, elimination, and analysis is needed. Stevens et al. analyze non-GFR-related factors associated with cystatin C and creatinine levels. The results will be useful in interpreting cystatin C levels in research and clinical practice.