Objective: To study the relationship between the prevalence of impaired glucose tolerance (IGT) and non-insulin-dependent diabetes mellitus (NIDDM) across populations of the Pacific Ocean region to assess whether variability in those two proportions followed some predictable pattern related to modernization of life-style and risk factor levels. Research Design and Methods: Prevalence estimates from studies with 75-g oral glucose loads and World Health Organization criteria were age standardized. Results: The linear correlation between IGT and NIDDM prevalence was poor (0.22 in men and 0.24 in women), although it was improved when the outlying data of Micronesian Nauruans and Australian Aborigines were excluded (r = 0.65, P < 0.01 in men and r = 0.54, P < 0.01 in women). However, an epidemicity index calculated as the percentage of total glucose intolerance (TGI) made up by IGT (i.e., IGT/TGI) had the strongest correlation with NIDDM prevalence (r = -0.81, P < 0.001 in men and r = -0.77, P < 0.001 in women) and also explained more of its variance, with no population having undue influence on the relationship. When IGT/TGI was plotted against NIDDM prevalence for the genetically homogeneous rural (relatively traditional) and urban (modernized) segments of five Pacific island populations, there was a clear tendency for IGT/TGI to decline as the prevalence of NIDDM increased in association with worsening risk factor levels. However, longitudinal data for the high prevalence population of Nauru demonstrated that at least in a stabilizing epidemic, changes in the prevalence of IGT and NIDDM may not be easily predictable. Conclusions: The epidemicity index may be useful as an indicator of the potential for higher future NIDDM prevalence in whole populations. Populations will probably equilibrate at a certain NIDDM prevalence dependent on the strength of their genetic susceptibility to the disease and their degree of exposure to adverse environmental risk factors, including modern diet, physical inactivity, and obesity.