In current clinical and research practice, the determination of diabetic status depends largely on plasma glucose levels 2 h after the ingestion of a standard 75-g glucose load, the oral glucose tolerance test (OGTT). The OGTT, however, remains inconvenient, not highly reproducible, and costly, especially for large-scale studies and population screening tests. Fasting plasma glucose (FPG) determinations are convenient, reliable, and valid measures of glucose intolerance, but the currently prescribed cut-off point of 140 mg/dl (7.8 mM) lacks sensitivity. We evaluated the reliability and validity of fasting plasma glucose (FPG) values compared with other measures of hyperglycemia for a diagnosis of diabetes in a population-based study of carbohydrate metabolism in Wadena, Minnesota, a community of predominantly northern European ancestry. As a part of this effort, a random sample of Wadena adults, stratified by age and gender, plus all known, previously diagnosed diabetics participated in 2 days of baseline testing and were followed prospectively and retested 5 years later. Cross-sectional analyses of baseline data are presented in this article. Diabetic status was ascertained by administering a standard OGTT according to National Diabetes Data Group (NDDG) specifications. Sensitivity and specificity levels obtained when using a FPG cut-off point of 6.4 mM were 95.2% and 97.4%, respectively. In study subjects with no known diagnosis of diabetes, the FPG cut-off point of 6.4 mM performed reasonably well with a sensitivity and specificity of 67.7% and 97.4%, respectively. (ABSTRACT TRUNCATED AT 250 WORDS)