The prevalence and incidence of type II diabetes are substantially higher in Mexican Americans than in non-Hispanic whites. This, combined with its earlier age of onset and the younger age structure of the Mexican American population, makes diabetes a major public health problem in this ethnic group and one which to a greater extent than in non-Hispanics strikes people in the prime of life. Both genetic and environmental factors undoubtedly contribute to this epidemic. The environmental factors would appear to be those commonly associated with the phenomenon of "modernization" or "westernization", namely, dietary change and adoption of sedentary lifestyles. As such, they are potentially modifiable and amenable to clinical and public health action. There is as yet no proof from clinical trials that modification of any or all of these environmental factors will in fact prevent diabetes. When clinical trials are performed, high-risk populations such as Mexican Americans should be considered as potential target populations, because of both the public health relevance and the lower sample size requirements in such populations. Further advances in identifying genetic susceptibles would also facilitate the design and execution of such trials. The increased risk of microvascular complications in Mexican American diabetics puts this population in "double jeopardy", i.e., they have a greater risk of acquiring diabetes in the first instance, and, having acquired it, they have a greater risk of suffering from its complications. It is still not clear whether the increased risk of complications in this ethnic group stems from an intrinsically more severe biological process or barriers to adequate medical care and/or poor compliance. Prospective data currently being gathered should help to resolve this issue. Assuming that medical care plays a role, a major opportunity--and challenge--exists to reduce substantially the morbidity, mortality, and economic burden associated with type II diabetes and its complications in the Mexican American population.