The American Diabetes Association (ADA) recently issued new guidelines for classification and diagnosis of diabetes mellitus. The main change is the decrease of the liminal value of the fasting plasma glucose level from 7.8 to 7.0 mmol/l. A fasting level of 6.1-6.9 mmol/l indicates impaired glucose tolerance (which eliminates the category 'impaired glucose tolerance', which was established on the basis of a slightly increased 2-hour glucose level after ingestion of 75 g glucose). Consequently, the ADA criteria render the oral glucose tolerance test (GTT) redundant for clinical practice. Given these criteria, the prevalence of diabetes mellitus among the general Dutch population will change only slightly, but the number of persons to be classified in a different category after their introduction is considerable: 39.2% of the ADA diabetics are not diabetics according to the current WHO classification, while 38.1% of the WHO diabetics are not diabetics according to the ADA criteria. The criteria established by the ADA accommodate clinical practice, in which the GTT is hardly used anymore. The WHO still has to decide about whether or not accepting the ADA guidelines.