Novel laboratory techniques such as the determination of proinsulin, insulin, C-peptide and insulin receptors have allowed further subclassification of diabetes mellitus today as representing a symptom rather than a disease. Numerous pathogenetic mechanisms of diabetes have been identified, many of these being genetically determined and this may extend possibilities for genetic counselling. As a geneticist's view, the pathogenetic concept of diabetes presented here is mostly confined to genetic mechanisms, leaving aside other influences on the development of diabetes such as hormones, viruses, nutrition and drugs. Mendelian inherited diabetic disorders are related to different pathogenetic principles where possible, being speculative in some cases.