OBJECTIVES The purpose of this study was to investigate the association of metabolic syndrome (MetS) with subclinical atherosclerosis, determined by ultrasound carotid intima-media thickness (CIMT) measurements, in young adults. BACKGROUND Metabolic syndrome is associated with subclinical atherosclerosis and increased cardiovascular risk in older and middle-aged adults; however, these associations have not been studied among young adults. METHODS Non-diabetic subjects from Bogalusa Heart Study, a longitudinal study of atherosclerosis in young adults, underwent B-mode ultrasonography of the carotid arteries. Metabolic syndrome was defined with the National Cholesterol Education Program Adult Treatment Panel III (MetS(NCEP)) and World Health Organization (MetS(WHO)) definitions. CIMT and MetS associations were evaluated with multivariable regression and area under receiver-operator characteristic curve (AUC) analyses. RESULTS Of 507 subjects (29% black, 39% male, mean [SD] age 32 [3] years), 67 (13%) had MetS(WHO) and 65 (13%) had MetS(WHO). Common (mean = 0.70 [0.11] mm vs. 0.66 [0.08] mm, p = 0.002) and internal CIMT (0.72 [0.21] mm vs. 0.68 [0.12] mm, p = 0.020) were higher among those with MetS(WHO) than those without MetS(NCEP). Common (0.69 [0.11] mm vs. 0.66 [0.081 rum, p = 0.020) and internal CIMT (0.73 [0.23] mm vs. 0.68 [0.12] mm, p = 0.012) also were higher among those with MetS(WHO) than those without MetS(WHO). Composite CIMT increased with the number of MetS components present (MetS,C,p r 0.997, p < 0-001; MetS(WHO) r = 0.946, p = 0.053). Metabolic syndrome,c,, (AUC 0.557, 95% confidence interval [CI] 0.513 -to 0.601) and MetS(WHO) (AUC = 0.539, 95% CI 0.495 to 0.584) both predicted composite CIMT >= 75th percentile. CONCLUSIONS In young adults, MetS is associated with increased atherosclerotic burden, and therefore, increased cardiovascular risk. These results support the importance of screening and early intervention in this population.