OBJECTIVE - Chlorinated persistent organic pollutants (POPs), endocrine disruptors accumulated in adipose tissue, were associated with diabetes and metabolic syndrome. Brominated flame retardants (BFRs), such as polybrominated diphenyl ethers (PBDEs) or polybrominated biphenyls (PBBs), are another class of POPs for which body burden is increasing. Cross-sectional associations of serum concentrations of BFRs with diabetes and metabolic syndrome were studied. RESEARCH DESIGN AND METHODS - In the National Health and Nutrition Examination Survey 2003-2004, 1, 367 adults were examined with respect to diabetes status. Five PBDEs and one PBB were selected, detectable in ≥60% of participants. For the outcome metabolic syndrome, we restricted the analysis to 637 participants with a morning fasting sample. RESULTS - Compared with subjects with serum concentrations below the limit of detection, prevalent diabetes had differing dose-response associations with serum concentrations of PBB- 153 and PBDE-153. Adjusted odds ratios across quartiles of serum concentrations for PBB-153 or PBDE-153 were 1.0, 0.7, 1.4, 1.6, and 1.9 (P for trend <0.01)and 1.0, 1.6, 2.6, 2.7, and 1.8 (P for quadratic term <0.01), respectively. PBB-153 was also positively associated with the prevalence of metabolic syndrome with adjusted odds ratios of 1.0, 1.5, 3.1, 3.1, and 3.1 (P for trend<0.01). As in its association with diabetes, PBDE-153 showed an inverted U-shaped association with metabolic syndrome. CONCLUSIONS - Pending confirmation in prospective studies, lipophilic xenobiotics, including brominated POPs stored in adipose tissue, may be involved in the pathogenesis of diabetes and metabolic syndrome.