Western guidelines have been used to diagnose diabetes mellitus in Asia. The increased availability of data from Asian populations, however, has made it apparent that modifications to western guidelines are needed when they are used in Asia. Both the American Diabetes Association and the World Health Organization have recently modified their diagnostic criteria for diabetes mellitus. The implications of these new criteria in Asia are discussed in this paper. The significance of using fasting plasma glucose measurements and/or oral glucose tolerance tests in the diagnosis of diabetes mellitus is analysed. A simple approach to diagnose diabetes mellitus in the Hong Kong Chinese population is also suggested.