This review summarizes aspects of the phenotypic expression, natural history, recognition, pathogenesis, and heterogeneous nature of maturity-onset diabetes of the young (MODY), which is inherited in an autosomal-dominant pattern. There are differences in metabolic, hormonal, and vascular abnormalities in different ethnic groups and even among White pedigrees. In MODY patients with low insulin responses, there are delayed and decreased insulin and C-peptide secretory responses to glucose from childhood or adolescence even before glucose intolerance appears, which may represent the basic genetic defect. When followed for decades, nondiabetic siblings have normal insulin responses. The fasting hyperglycemia of some MODY patients has been treated successfully with sulfonylureas for up to 30 yr. In a few patients, after years or decades of diabetes, the insulin and C-peptide responses to glucose are so low that they resemble those of early insulin-dependent diabetes mellitus. The progression of the insulin secretory defect over time distinguishes between these two types of diabetes. In contrast are patients from families who have very high insulin responses to glucose, despite glucose intolerance and fasting hyperglycemia similar to that seen in patients with low insulin responses. In many of these patients, there is in vivo and in vitro evidence of insulin resistance. Whatever its mechanism, the compensatory insulin responses to nutrients must be insufficient to maintain normal carbohydrate tolerance. This suggests that diabetes occurs only in those patients who have an additional islet cell defect, i.e., insufficient beta-cell reserve and secretory capacity. In a few MODY pedigrees with high insulin responses to glucose and lack of evidence of insulin resistance, a structurally abnormal mutant insulin molecule that is biologically ineffective is secreted. No associations have been found between specific HLA antigens and MODY in White, Black, and Asian pedigrees. Linkage studies of the insulin gene, insulin-receptor gene, erythrocyte/HepG2 glucose-transporter locus, and apolipoprotein B locus have shown no association with MODY. Vascular disease may be as prevalent as in conventional non-insulin-dependent diabetes mellitus. Because of autosomal-dominant transmission and penetrance at a young age, MODY is a good model for further investigations of etiologic and pathogenetic factors in non-insulin-dependent diabetes mellitus, including the use of genetic linkage strategies to identify diabetogenic genes.