OBJECTIVE - It is controversial whether the clustering of certain metabolic abnormalities should be separately designated as the metabolic syndrome. We operationalized the "syndrome" concept and tested whether the metabolic syndrome was compatible with these operational constructs. RESEARCH DESIGN AND METHODS - The baseline cross-section of the Multi-Ethnic Study of Atherosclerosis recruited a population-based cohort of 6,781 individuals, aged 45-84 years, from six communities in the U.S. Metabolic syndrome components (waist circumference, blood pressure, fasting serum HDL cholesterol, triglycerides, and plasma glucose), homeostasis model assessment (HOMA) of insulin resistance (fasting glucose X insulin), and intimal-medial thickness (IMT) in the common and internal carotid arteries by B-mode ultrasound were measured. RESULTS - Higher syndrome component count is associated with higher HOMA levels (trend P < 0.001). Given the prevalence of individual components, the nonprevalence of any component or the co-prevalence of four or five components is greater than expected (χ2 P < 0.001). After accounting for the additive association of each component, the current definition of metabolic syndrome (co-prevalence of three or more components) does not have supra-additive association with thicker IMT in the common carotid (men: P = 0.075, women: P = 0.949) or internal carotid artery (men: P = 0.106, women: P = 0.121). CONCLUSIONS - The metabolic syndrome did not have supra-additive association with IMT, but its components clustered greater than chance expectation and a higher component count was associated with greater insulin resistance. The metabolic syndrome was compatible with two of three "syndrome" constructs tested.