Background: A common belief is that one quarter to one third of all diabetes cases remain undiagnosed. However, such prevalence estimates may be overstated by epidemiologic studies that do not use confirmatory testing, as recommended by clinical diagnostic criteria. Objective: To provide national estimates of undiagnosed diabetes by using a confirmatory testing strategy, in line with clinical practice guidelines. Design: Cross-sectional study. Setting: National Health and Nutrition Examination Survey results from 1988 to 1994 and 1999 to 2014. Participants: U.S. adults aged 20 years and older. Measurements: Confirmed undiagnosed diabetes was defined as elevated levels of fasting glucose (>/=7.0 mmol/L [>/=126 mg/dL]) and hemoglobin A1c (>/=6.5%) in persons without diagnosed diabetes. Results: The prevalence of total (diagnosed plus confirmed undiagnosed) diabetes increased from 5.5% (9.7 million adults) in 1988 to 1994 to 10.8% (25.5 million adults) in 2011 to 2014. Confirmed undiagnosed diabetes increased during the past 2 decades (from 0.89% in 1988 to 1994 to 1.2% in 2011 to 2014) but has decreased over time as a proportion of total diabetes cases. In 1988 to 1994, the percentage of total diabetes cases that were undiagnosed was 16.3%; by 2011 to 2014, this estimate had decreased to 10.9%. Undiagnosed diabetes was more common in overweight or obese adults, older adults, racial/ethnic minorities (including Asian Americans), and persons lacking health insurance or access to health care. Limitation: Cross-sectional design. Conclusion: Establishing the burden of undiagnosed diabetes is critical to monitoring public health efforts related to screening and diagnosis. When a confirmatory definition is used, undiagnosed diabetes is a relatively small fraction of the total diabetes population; most U.S. adults with diabetes (about 90%) have received a diagnosis of the condition. Primary Funding Source: National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases and National Heart, Lung, and Blood Institute.