OBJECTIVE To compare the prevalence in metabolic syndrome (MetSyn) between 1988–1994 and 1999–2006 among U.S. adults of different races or ethnicities. RESEARCH DESIGN AND METHODS Analysis of data on 6,423 adult men and nonpregnant women aged ≥20 years from Third National Health and Nutrition Examination Survey (NHANES III) and 6,962 participants from the combined NHANES 1999–2006 were done. The revised National Cholesterol Education Program Adult Treatment Panel III definition was used to calculate MetSyn. RESULTS Both the unadjusted prevalence (27.9 ± 1.1% to 34.1 ± 0.8%, P < 0.001) and age-adjusted prevalence (29.2 ± 1.0% to 34.2 ± 0.7%, P < 0.001) increased from NHANES III to NHANES 1999–2006, respectively. Although MetSyn prevalence was highest in Mexican Americans, significant increases in prevalence occurred among non-Hispanic whites and non-Hispanic blacks, especially among younger women. CONCLUSIONS The persistent increase of MetSyn among U.S. adults is a serious public health concern because it raises the likelihood of increased prevalence of type 2 diabetes.