Type 2, non-insulin-dependent, diabetes is a disease of glucose homeostasis involving up to 5% of the adult population. The percentage of the population with glucose intolerance is even greater, 10%. These later patients are not diagnosed as "diabetics", but 50% of them have non-insulin-dependent diabetes. Globally, 10% of the adult population has or will have Type 2 diabetes, a major health care problem. Characteristically, in patients with Type 2 diabetes, pancreatic insulin secretion is deficient, liver glucose production is increased during the post-absorption period and peripheral glucose consumption, particularly in striated muscles, is decreased due to insulin resistance. There has been much progress in our understanding of the pathogenic mechanisms involved. When clinical manifestations have become apparent, the relative roles of defective insulin secretion and insulin resistance are difficult to distinguish. However, in persons with oral glucose intolerance or in persons with a high risk of developing Type 2 diabetes, these two mechanisms are more easily differentiated. High risk patients can be identified on the basis of our knowledge of genetic factors in Type 2 diabetes. The incidence of Type 2 diabetes is considerably increased in subjects with two diabetic parents. In addition to genetic factors, environmental factors also influence the development of non-insulin-dependent diabetes, contributing to the multifactorial nature of Type 2 diabetes. In order to establish the relative importance of these different factors, it is useful to define the different stages with characteristic degrees of metabolic disorder, insulin secretion abnormalities and insulin secretion.(ABSTRACT TRUNCATED AT 250 WORDS)