OBJECTIVE:For epidemiological purposes, it has now been recommended that a fasting plasma glucose value of 7.0 mmol/l can be used to diagnose diabetes, instead of a 2-h value of 11.1 mmol/l. This study assesses the impact of making this change on the prevalence of diabetes and on the phenotype of individuals identified. RESEARCH DESIGN AND METHODS:Data were collated from nine population based southern hemisphere studies in which a 75-g oral glucose tolerance test was performed. Comparisons were made between the prevalence derived from fasting values only and the prevalence derived from 2-h values only. Cardiovascular risk was assessed in all individuals. RESULTS:There were 20,624 subjects in the nine surveys of whom 1,036 had previously diagnosed diabetes and 1,714 had newly diagnosed diabetes, according to either fasting or 2-h glucose. The differences in prevalence within each population resulting from changing the diagnostic criteria ranged from +30 to -19% (relative difference) and +4.1 percentage points to -2.8 percentage points (absolute difference). BMI was the most important determinant of disagreement in classification. A total of 31% of those individuals who were diabetic on the fasting value were not diabetic on the 2-h value, and 32% of those with diabetes on the 2-h value were not diabetic on the fasting value. Apart from obesity, there were no differences in cardiovascular risk between those identified by the fasting and the 2-h values. CONCLUSIONS:Changing the diagnostic criteria is likely to have variable and sometimes quite large effects on the prevalence of diabetes in different populations. Furthermore, the fasting criterion identifies different people as being diabetic than those identified by the 2-h criterion.