Many genetic syndromes are associated with the development of diabetes mellitus, as a result of insulin resistance, low plasmatic insulin levels and vitamin deficiencies. In some of them diabetes occurs as one of the main features of the illness, as in the Wolfram, Wolcott-Rallison, Rogers, Donohue and Rabson-Mendehall syndromes. In others, diabetes occurs as an additional finding, not always present, as in the Prader-Willi, Bardet-Biedl, Alström, Turner, Down and Klinefelter syndromes. Since there are many syndromes, their clinical manifestations are variable. The investigation of genetic syndromes is recommended in all diabetic patients who present with neurosensorial deafness, optic atrophy, megaloblastic anaemia, diabetes insipidus, maternal family history of diabetes, urinary and respiratory changes, neurological disorders, myopathy and dysmorphic features on the physical examination. Patients with a genetic syndrome commonly associated with diabetes must also be investigated. The availability of genetic tests, prenatal diagnosis and the early identification of the manageable complications associated with these syndrome justify the review of this subject.