Diabetes mellitus is usually subdivided into type I (insulin-dependent) and type II (relative insulin shortage and reduced sensitivity to insulin). Diabetes may also be related to pregnancy, malnutrition, pancreatic disease, pharmaceuticals, endocrine diseases and hereditary disorders. The hereditary diseases which may be associated with diabetes mellitus or impaired glucose tolerance can be subdivided into syndromes (such as maternally inherited diabetes and deafness, Down, Turner and Klinefelter syndrome), metabolic diseases (like cystic fibrosis and haemochromatosis) and endocrine diseases (like polyglandular autoimmune insufficiency syndrome and familial phaeochromocytoma). Although diabetes mellitus as part of a hereditary disorder is infrequent, the possibility should be kept in mind with a view to a correct diagnosis. In patients with diabetes mellitus a hereditary disorder may be involved, while patients with a hereditary disorder run a higher risk of developing diabetes mellitus.