Impaired glucose tolerance (IGT) constitutes two-thirds of all glucose intolerance in the United States and is a major risk factor for diabetes. Despite these findings, the clinical and epidemiological significance of IGT has not been well investigated. The Second National Health and Nutrition Examination Survey, a cross-sectional study in which 75-g 2-h oral glucose tolerance tests (OGTTs) were performed, has provided an opportunity to examine the characteristics of IGT in the U.S. population. Data from the survey have been extrapolated to represent all U.S. residents. The findings indicate that approximately 11.2% of Americans aged 20-74 yr have IGT compared to 6.6% with diabetes. Rates of IGT increased with age for White men and women and Black men but declined for Black women greater than 54 yr of age, possibly because greater obesity in Black women precipitated earlier conversion of IGT to diabetes. The distribution of 2-h glucose values showed IGT to be part of a continuum of glucose intolerance extending from normal to diabetes. Individuals with IGT had rates of risk factors for non-insulin-dependent diabetes (age, plasma glucose, past obesity, family history of diabetes, physical inactivity) that were intermediate between those of individuals with normal glucose tolerance and those with diabetes, although current obesity was similar for IGT and diabetes. The proportion of people with medical histories of diabetes-related conditions did not differ between IGT and normal glucose tolerance. However, several cardiovascular findings were more prevalent in individuals with IGT than in those with normal glucose tolerance, including hypertension, serum cholesterol, angina, abnormal heart findings, and medical history of arteriosclerosis and stroke. Both obesity and reported family history of diabetes were associated with higher rates of IGT, with the effect of weight gain on the prevalence of IGT occurring at lower levels than for diabetes.