Serum cystatin C has been associated with cardiovascular disease. We investigated whether cystatin C concentration is associated with the metabolic syndrome and with other cardiovascular risk factors in a hypertensive population. In this cross-sectional study, we prospectively included 611 essential hypertensive patients during a 12-month period. Cystatin C concentration was measured by nephelometry. The metabolic syndrome was present in 46% of the patients. Cystatin C was significantly higher in patients with the metabolic syndrome (0.94 +/- 0.27 mg/L) than in those without (0.87 +/- 0.23 mg/L) (P < .0001). Pearson partial correlation analysis showed a significant correlation between cystatin C and body mass index (r = 0.240; P = .001); waist circumference (r = 0.173; P = .012); microalbuminuria (r = 0.273; P < .0001); triglycerides (r = 0.138; P = .047); C-reactive protein (r = 0.190; P = .006); uric acid (r = 0.284; P < .0001); age (r = 0.409; P < .0001); and glomerular filtration rate (GFR) (r = -0.638; P < .0001). Multivariate analysis showed that GFR (B = -0.0061; 95% confidence interval [CI], -0.0073 to -0.0049; P < .0001), age (B = 0.0023; 95% CI, 0.0005-0.0041; P = .009), microalbuminuria (B = 0.0005; 95% CI, 0.0002-0.0007; P < .0001), uric acid (B = 0.0252; 95% CI, 0.0085-0.0418; P = .003), body mass index (B = 0.0051, 95% CI, 0.0012-0.0089; P = .011), and C-reactive protein (B = 0.0048; 95% CI, 0.0015-0.0082; P = .005) were independent determinants of cystatin C concentration. Measuring cystatin C concentration in hypertensive patients may be useful for evaluating their cardiovascular risk profile.