Diabetes and its complications are major causes of morbidity and mortality in the United States and contribute substantially to health-care costs. Data from the National Health Interview Survey (NHIS) and the Behavioral Risk Factor Surveillance System (BRFSS) have documented steady increases in the prevalence of diabetes. However, these surveys rely only on self-reports of previously diagnosed diabetes and cannot measure the prevalence of undiagnosed diabetes. The change in prevalence demonstrated by these data might reflect other factors such as enhanced detection rather than true increases. The National Health and Nutrition Examination surveys (NHANES) are the only nationally representative surveys that examine both diagnosed and undiagnosed diabetes. During 1976-1980 (NHANES II) and 1988-1994 (NHANES III), the overall combined prevalence of diabetes (diagnosed and undiagnosed on the basis of fasting glucose) increased. This report presents data on prevalence of diagnosed and undiagnosed diabetes and impaired fasting glucose from NHANES 1999-2000 and NHANES III (1988-1994). The findings indicate that diabetes and impaired fasting glucose continue to affect a major proportion of the U.S. population. An estimated 29 million (14.4%) persons aged >/=20 years had either diagnosed diabetes, undiagnosed diabetes, or impaired fasting glucose; 29% of diabetes cases were undiagnosed. Persons can reduce their risk for diabetes through weight management and physical activity.