Although metabolic anomalies are often seen in advanced chronic kidney disease (CKD), their presence in more mild states is unknown. We studied 6722 participants in the Third National Health and Nutrition Examination Survey, dividing them into three mutually exclusive groups consisting of those having a normal or mildly reduced estimated glomerular filtration rate (eGFR) (Modification of Diet in Renal Disease (MDRD) formula), those with normal or elevated serum cystatin C, and those with clinically relevant moderate or severely reduced eGFR (stage 3 or 4 of CKD). The prevalence of several metabolic abnormalities associated with moderate to advanced CKD was determined after standardization for age, race-ethnicity, and gender. In the absence of stage 3 or 4 CKD, patients with elevated serum cystatin C had a higher prevalence of low hemoglobin and elevated uric acid, homocysteine, phosphorus, fibrinogen, and C-reactive protein than patients with a normal serum cystatin C. Our results show that in adults with normal or mildly reduced eGFR, elevated serum cystatin C is associated with an increased prevalence of metabolic abnormalities traditionally found in moderate or severe CKD. Elevated serum cystatin C may identify patients with 'preclinical' kidney disease not detected by traditional serum creatinine measurements.