In July 2009, an international committee announced a new diagnostic criterion for diabetes based on hemoglobin Alc (HbA1c) values. Our objective was to estimate how the new diabetes diagnostic criterion will affect the prevalence of diabetes among different race, age, and gender subpopulations, compared to the previously used fasting plasma glucose (FPG) criterion. We analyzed nationally representative data from The National Health and Nutrition Examination Survey (NHANES), aggregated from 1999 to 2006. We estimated the prevalence of known diabetes (prevalence static across either diagnostic criterion), unknown, and no diabetes (prevalence variable by criterion). We tested statistical significance of prevalence differences for unknown diabetes between the prior diagnostic criterion--FPG of at least 126 mg/dL--and the new diagnostic criterion--HbA1c of at least 6.5%--using conditional logistic regression. We further tested the association of these differences with demographic factors. The new HbA1c diagnostic criterion differentially affects different racial/ethnic groups. For non-Hispanic whites, the prevalence of undiagnosed diabetes was more than halved from 2.6% (95% confidence interval [CI], 2.2-3.1) with FPG diagnosis to 1.3% (95% CI, 1.0-1.7), P<.001 with HbAic diagnosis. For Hispanics and non-Hispanic blacks, the differences in prevalence by the 2 criteria were smaller and nonsignificant. Racial differences by diagnostic criteria were most pronounced among people aged over 55 years. Overall, the new definition of diabetes differentially affects ethnic groups, especially for older people. If the new criterion is widely adopted, over time, we may see an apparent widening of racial/ethnic disparities in diabetes prevalence.