The search for genetic markers has established that idiopathic diabetes mellitus is a genetically heterogeneous group of disorders that have glucose intolerance in common. The types of genetic markers--subclinical linkage, and association--and how they are utilized to delineate the genetic basis of the diabetic syndrome are discussed. The use of such markers as insulin levels, pancreatic islet cell antibodies, and HLA antigens has permitted the separation of insulin-dependent (juvenile) and not insulin-dependent (maturity) diabetes. Such studies have also started to reveal heterogeneity within these broad groups of insulin-dependent and not insulin-dependent types. This extensive heterogeneity has major implications for understanding the pathogenesis and genetics of diabetic mellitus and is of potentially great clinical significance, since the natural history and complications may well differ between these different disorders.